Q.1.     What is Chemistry?

Ans.     Chemistry is that branch of science which deals with the study of the composition, structure, properties and changes of matter, the laws and principles which govern these changes.

Q.2.     What is an atom?

Ans.     The smallest particle of an element that retains the chemical identity of the element and can take part in a chemical reaction is called an atom. It may or may not exist independently. For example, the atoms of He and Ne, have independent existence, while atoms of H, O and N do not exist independently.

Q.3.     Define molecule?

Ans.     The smallest particle of a pure substance which can exist independently is called molecule. Molecules of elements may contain one, two or more of the same type of atoms. For example, He, O2, Cl2, P4, S8, etc. while the molecules of compound contain more than one type of atoms. For example, H2O, HCl, NH3, H2SO4.

Q.4.     What is meant by atomicity?

Ans.     The number of atoms present in a molecule is called its atomicity. Thus molecules may be monoatomic, diatomic and triatomic, etc., depending upon they contain one, two and three atoms respectively.

Q.5.     What is an ion?

Ans.     Species that carry either positive or negative charge is called an ion. An ion with a positive charge is called a cation and negatively charged ion is called an anion. Na+, K+, Ca2+, Al3+ are examples of cations. Cl, Br, S2- are examples of anions.

Q.6.     What is meant by molecular ions?

Ans.     A molecular species having positive or negative charge is called molecular ion, e.g., . These ions can be generated by passing high electron beam as -particles or X-rays through a gas.

Q.7.     Why the formation of a cation is an endothermic process?

Ans.     Energy is always required when an atom of an element loses one or more electrons to form cations. Therefore, the formation of cations is an endothermic process.

Q.8.     Why the formation of an anion is a exothermic process?

Ans.     Energy is usually when an electron is added to the isolated neutral atom to form a uninegative ion. Therefore, the formation of an anion is exothermic process.

Q.9.     Define relative atomic mass?

Ans.     The mass of an element as compared to the mass of an atom of carbon taken as 12 is called relative atomic mass.

Q.10.   What is atomic mass unit?

Ans.     The unit used to express the relative atomic mass is called atomic mass unit and is 1/12th of the mass of one carbon atom. An amu equals 1.66054 x 10-24 g or 1.66054 x 10-27 Kg. it is obtained by dividing the unity by Avogadro’s number (6.02 x 1023). In these units the mass of the hydrogen -1 atom is 1.0078 amu and that of oxygen – 16 is 15.9949 amu.

Q.11.   what is meant by isotopes?

Ans.     Atoms of the same element that have same atomic number but different atomic masses are called isotopes Carbon has three isotopes written as and  are expressed as C-12, C-13 and C-14.

Q.12.   why the isotopes have the same chemical properties?

Ans.     The isotopes of an element have same chemical properties because they have equal number of electrons and have the same type of arrangement of electrons at the periphery of their atoms.

Q.13.   the atomic masses may be in fractions. Why?

Ans.     The atomic masses depend upon the number of possible isotopes and their natural abundance. Isotopes have fractional atomic masses. Therefore, the average atomic masses of elements may be in fractions.

Q.14.   How the average atomic mass of an element is determined from isotopic masses?

Ans.     The average atomic mass of an element is found by multiplying the abundance of each isotope by its atomic mass and summing these products.

Q.15.   No individual neon atom in the sample of the element has a mass of 20.18 amu. Why?

Ans.     Neon has three isotopes of atomic masses 20, 21 and 22 with relative abundances of 90.92%, 0.26% and 8.82% respectively. The average atomic mass of neon is determined to be 20.18 amu. Thus no individual neon atom in the sample of the element has a mass of 20.18 amu.

Average atomic mass =  = 20.18 amu

Q.16.   What is mass spectrometer?

Ans.     Mass spectrometer is an instrument which is used to determine the direct and accurate masses of different isotopes of an element.

Q.17.   Write down briefly the principle of the mass spectrometer?

Ans.     The vapour or gaseous sample of a substance is introduced into the ionization chamber and bombarded by a stream of high-energy electrons to produce positive ions. These ions are accelerated toward a negatively charged wire grid and then a narrow beam of ions is allowed to pass through a strong magnetic field of strength (H), which deflects the ions into a curved path. The extent of deflection depends on the (m/e) values and thus they are separated on the basis of their (m/s) values. The ions of varying (m/e) values are selected to enter the electrometer, which gives the relative abundance of ions of a definite m/e value.

Q.18.   Why positively charged ions of isotopes are passed  through magnetic field in the mass spectrometer?

Ans.     The positively charged ions are passed between the poles of a magnet, which deflects the ions into a curved path. The extent of deflection depends on the m/e values and thus the ions are separated. Therefore, the positively charged ions of isotopes are passed through magnetic field in the mass spectrometer to separate the isotopes of the elements.

Q.19.   What is the function of electrometer in mass spectrometer?

Ans.     Electrometer is also called as ion collector, develops the electrical current which is proportional to the number of ions falling on it and thus the current strength in each case gives the relative abundance of each of the isotopes. The relative abundance of these isotopes is recorded in the form of peaks on the graph of the relative abundance of the isotopes versus the mass number.

Q.20.   What is mass spectrum of mass spectrograph?

Ans.     A graph plotted between m/e of the positive ions as abscissa (x-axis) and relative abundance of the isotopes as ordinate (y-axis) is called a mass spectrum.

Q.21.   Define empirical formula and how it is related to molecular formula?

Ans.     A chemical formula that gives only the relative number of atoms of each element present in a compound is called empirical formula.

Molecular formula = n x empirical formula

n =

the empirical formula of benzene (C6H6) is CH and that of glucose (C6H12O6) is CH2O.

Q.22.   Define molecular formula with examples?

Ans.     A chemical formula that indicates actual number of atoms of each element in a molecule is called molecular formula. For example, the molecular formula of benzene is C6H6 while that of glucose is C6H12O6.

Q.23.   Molecular formula is multiple of empirical formula. Give an example.

Ans.     The empirical formula gives only the simplest whole number ratio of atoms of each element in a compound, while molecular formula indicates the actual number of atoms of each element in a molecule. Therefore, empirical formula is multiplied with a suitable integer ‘n’ (Which is the ratio of molecular mass and empirical formula mass of a substance) to get the molecular formula. For example, benzene, 6CH= C6H6; glucose, 6CH2O = C6H12O6

Q.24.   What is meant by the mole?

Ans.     The basic SI unit for the amount of substance is mole, abbreviated mol and is a counting unit like a dozen. It is a very large counting unit and is equal to 6.02 x 1023 atoms, ions or molecules under consideration. A mole is defined as the amount of matter that contains as many objects (atoms, ions, molecules or whatever objects as under consideration) as the number of atoms in exactly 12 g of 12C.

Q.25.   Define gram-atom, gram molecule, gram formula and ionic mass?

Ans.     The atomic mass of an element expressed in grams is called a gram atom or mole of that element. The molecular mass of a substance expressed in grams is called gram molecule or simple the mole of that substance.

The formula unit mass of an ionic compound expressed in grams is called gram formula of that substance. The ionic mass of an ionic species expressed in grams is called one gram ion or one mole of ions.

16.0 g oxygen = 1 gram atom of O      = 1mole of O atom

32.0 g of oxygen = 1 gram molecule of O2 = 1 mole of O2

58.5 g of NaCl = 1 gram formula of NaCl = 1 mole of NaCl

17.0 g OH = 1 gram ion of OH = 1 mole of OH

Q.26.   Define molar mass?

Ans.     The atomic mass, molecular mass, formula mass or ionic mass of the substance expressed in grams is called molar mass of the substance, e.g., 23g of Na is the molar mass of Na.

Q.27.   Define Avogadro’s umber with examples?

Ans.     The number of atoms, molecules or ions in one gram atom of an element, one gram molecule of a compound or one gram ion of a substance, respectively is called Avogadro’s number. It is denoted by NA and its value is found to bee 6.02 x 1023 . for example

1.008 g of hydrogen    =          1 mole of H     = 6.02 x 1023 atoms of H

18.0 g of H2O             =          1 mole of water           = 6.02 x 100 molecule of H2O

96 g of SO                =          1 mole of SO  = 6.02 x 1023 ions of SO

Q.28.   Define molar with examples?

Ans.     The volume of 22.414 dm3 occupied by one mole of any gas at STP is called molar volume and it is true only when the gas is ideal.

2.016 g of H2   =          1 mole of H2    =          22.414 dm3 of H2 at STP

16 g of CH4     =          1 mole of CH4             =          22.414 dm3 of CH4 at STP

Q.29.   23g of Na and 238g of uranium have equal number of atoms in them. Justify it.

Ans.     23g of Na equals 1 mole of Na and 238g of uranium also equals 1 mole of U. one mole of each element contained in it 6.02 x 1023 atoms. Therefore, 23g of Na and 238g of U have equal number of atoms in them.

Q.30.   Mg atom is twice heavier than that of carbon atom. Justify.

Ans.     The atomic mass of Mg is 24 amu which is twice in mass as compared to the atomic mass of C i.e. 12 amu. Therefore, Mg atom is twice heavier than that of carbon.

Q.31.   180g of glucose and 342g of sucrose have same number of molecules, but different number of atoms present in them. Justify it.

Ans.     180g of glucose (C6H12O6) equals one mole of glucose and also 342g of sucrose (C12H22O11) equals one mole of sucrose. One mole of any substance contain equal number of molecules. i.e. 6.02 x 1023 atoms. Therefore, 180g of glucose and 342g of sucrose have different number of atoms.

Q.32.   4.6g of H2SO4, when completely ionized in water have equal number of positive and negative charges, but number of positively charged ions are twice the number of negatively charged ions.

Ans.     4.9g of H2SO4 on complete ionization in water gives 0.05 moles of H2SO4. 0.05 moles of H2SO4 will yield 0.1 mole or 0.1 x 6.02 x 1023H+ and 0.05 moles or 0.05 x 6.02 x 1023 SO  which shows that the number of positively charged ions are twice the number of negatively charged ions. Total positive charges will be 0.1 x 6.02 x 1023 and total negative charges will be 0.1 x 6.02 x 1023 (because each SO has two negative charges). Thus 4.9g of H2SO4 on complete ionization have equal number of positive and negative ions.

Q.33.   One mg of K2CrO4 has thrice the number of ions than the number of formula units when ionized in water. Justify it.

Ans.     K2CrO4 is completely ionized is water as follows:

K2CrO4          2k+ + CrO42-

One formula unit of K2CrO4 produces two K+ and oneCrO42-  ions. Thus total number of ions produced are thrice the number of ions as compared to its formula unit. So, one mg of K2CrO4 has thrice the number of ions than the number of formula units when ionized in water.

Q.34.   Two grams of H2, 16g of CH4 and 32g of CO2 occupy separately the volume of 22.414 dm3, although the sizes and masses of three gases are very different from each other.

Ans.     The sizes and masses of molecules of different gases do not affect the volume. One mole of any gas at STP occupies a volume of 22.414 dm3.therefore 2g of H2, 16g of CH4 and 44g of CO2, thus each gas having 1 mole , occupy separately the volume of 22.414 dm3.

Q.35.   One mole of H2SO4 should completely react with two moles of NaOH. How does Avogadro’s number help to explain it.

Ans.     H2SO4 + 2NaOH           Na2SO4 + 2H2O

One mole of H2SO4 produces 2 moles of H+ ions or 2 x 6.02 x 1023 H+ ions. Thus it requires 2 moles of OH- or 2 x 6.02 x1023 OH  ions from a base for complete neutralization. So, this is the Avogadro’s number, 6.02 x 1023, which help to explain this reaction.

Q.36.   NaCl has 58.5 amu as formula mass and not the molecular mass. Justify it.

Ans.     NaCl is an ionic compound and does not exist in molecular form. Therefore, NaCl is a formula unit and has 58.5 amu as formula mass and not the molecular mass.

Q.37.   Calculate the mass in grams of 10-3 moles of H2O.

Ans.     Mass of H2O = No. of moles x molar mass

                        = 10-3 mole x 18g mol-1 = 1.8 x 10-2 g = 0.018g

Q.38.   How does Avogadro’s number relate to the masses of chemical substances?

Ans.     Atomic mass, molecular mass or ionic mass of a substance expressed in grams is called a gram mole or simply mole, which contains 6.02 x 1023 (Avogadro’s number) atoms, molecules or ions in it. Thus Avogadro’s number relates to the masses of chemical substance as shown by the example:

6.02 x 1023 atoms of Na = 1 mole of Na = 23g of Na

Q.39.   Prove that one mole of each N2, CO2, and H2 contain equal number of molecules.

Ans.     One mole of each substance contains 6.02 x 1023 molecular in it, so one mole of each N2, CO2 and H2 contains 6.02 x 1023  molecular in them. Which are equal number of molecules.

Q.40.   What is Stoichiometry?

Ans.     The study of quantitative relationship between the amounts of reactants and products in a  balanced chemical equation is called Stoichiometry.

Q.41.   What scientific law is used in the process of balancing chemical equation?

Ans.     Law of conservation of mass.

Q.42.   In balancing equations, why should not subscripts in chemical formulas be changed?

Ans.     Subscripts in chemical formulas should not be changed when balancing equations because changing the subscripts change the identity of the compound.

Q.43.   Why is it essential to use balanced chemical equation in showing stoichiometry problems?

Ans.     The mole ratio implicit in the coefficients of a balanced chemical equation are essential for solving stoichiometry problems. If the equation is not balanced, the mole ratio will be incorrect and lead to erroneous calculated  amounts of reactions and products.

Q.44.   What is limiting reagent? How it helps to control the reaction?

Ans.     The reactant that is completely consumed in a reaction is called limiting reagent because it determines, or limits the amount of product formed. Once the limiting reagent is consumed, the reaction stops and no additional product is formed. Hence the limiting reagent helps to control the reaction.

Q.45.   What is the relationship between the formula mass of a substance and its molar mass?

Ans.     The formula mass of a substance in amu has the same numerical value as the molar mass expressed in grams.

Q.46.   Concept of limiting reactant is not applicable to the reversible reactions. Explain it?

Ans.     The concept of limiting reactant is not applicable to the reversible reactions, because certain amounts of each reactant left behind at the equilibrium state and thus any reactant is not completely consumed as restricted by the quantity of the limiting reactant in a reaction.

Q.47.   11g of carbon is reacted with 32g of O2 to give CO2. Which is the limiting reactant.

Ans.     C + O2       CO2

One mole of C (12g) allow it to react with one mole of O2 (32g) to give 1 mole of CO2 (44g) according to the balanced equation. Since less amount of carbon is present as compared to oxygen, so carbon is the limiting reactant.

Q.48.   Distinguish between actual and theoretical yield?

Ans.     The amount of product actually obtained in a reaction is called the actual yield. The amount of the products calculated from the balanced chemical equation is called the theoretical yield. The actual yield is almost always less than the theoretical yield.

Q.49.   How can the efficiency of a chemical reaction be expressed?

Ans.     The efficiency of a reaction is expressed by comparing the actual and theoretical yields in the form of percentage yields.

% yield = Actual yield / Theoretical yield x 100

Q.50.   Why does the actual yield is almost always less than the theoretical yield.

Ans.     This is due to the following reasons.

  1. The mechanical loss of products during the processes like filtration, distillation, washing, drying an crystallization.
  2. The reactants may react in a way different from that desired i.e., side reactions.
  3. Reversibility of reactions.

Q.51.   What isotope is used as the standard in establishing the atomic mass scale?

Ans. The isotope of carbon 6C12 is used in atomic mass scale.

Q.52.   The atomic mass of Mg is reported as 24.3, yet no atom of Mg has the mass of 24.3 amu. Explain.

Ans.     Atomic masses are average atomic masses, the sum of the mass of each naturally – occurring isotope of an element multiplies of fractional abundance. Each Mg atom will have the mass of one of the naturally occurring isotopes, while the ‘atomic mass’ is an average value.

Q.53.   the molecular formula of aspartame, the artificial sweetener, is Cr4H18N2O5. (a) what is the molar mass of aspartame (b) How many moles of aspartame are present in 1.00mg of aspartame.

Ans.     (a) Molar mass of aspartame = (12 x 14) + (1 x 18) + (2 x 14) + (5 x 16) = 294g mol-1

(b) Moles of aspartame = Mass/molar mass = 1 x 10-3g/ 294g mol-1 = 3.4o x 10-6 mol

Q.54.   One mole of H2O has two moles of bonds, three moles of atoms, ten moles of electrons and twenty eight moles of the total fundamental particles present in it. Justify?

Ans.     One molecule of H – O – H has two bonds, three atoms (2 H atoms and one O atom), ten electrons (two H electrons and 8 oxygen electrons) and twenty eight fundamental particles (10 electrons + 10 protons and 8 neutrons) in it. Therefore, one mole of H2O has two moles of bonds, three moles of atoms, 10 moles of electrons and 28 moles of the total fundamental particles in it.

Q.55.   N2 and CO have same number of electrons, protons and neutrons justify?

Ans.     N2 (consisting of 2N atoms) have 14 electrons (7+7), 14 protons and 14 neutrons. In CO, C has 6 electrons, 6 protons and 6 neutrons and O atom has 8 electrons, 8 protons and 8 neutrons. Thus CO like N2 has also 14 electrons, 14 protons and 14 neutrons. Thus N2 and CO have same number of electrons, protons and neutrons.

Q.56.   How many Na+ and Cl ions are present in 5.58 g of NaCl?

Ans.     The formula mass NaCl is 58.5 amu. 5.58g of NaCl equals 0.1 mole of NaCl. Thus 0.1 mole of NaCl contains 6.02 x 1022 particles.

Q.57.   What are the limitations of chemical equation?

Ans.     A chemical equation does not tell about the conditions, mechanism and the rate of reaction.

Q.58.   Name the laws which should be obeyed while doing stoichiometric calculations?

Ans.     1. Law of conservation of mass.

            2. Law of definite proportions.

Q.59.   in which absorber H2O vapour and CO2(g) are absorbed?

Ans.     H2O and CO2 gases are absorbed in Mg (ClO4)2 and 50% KOH respectively.

Q.60.   What is the justification of two strong peaks in the mass spectrum for bromine while for iodine only peak at 127 amu is indicated?

Ans.     The two strong peaks in the mass spectrum for bromine justify that bromine has two isotopes; while for iodine only one isotope i.e. iodine is monoisotopic.

Q.61.   What are assumptions in stoichiometric calculations?

Ans.     The following are the assumptions in stoichiometric calculations.

  1. Reactants are completely converted into the products.
  2. No side reaction occurs.
  3. The law of conservation of mass and the law of definite proportions are obeyed.

Q.62.   What is the number of H+ ions in 9.8 of H3PO4?

Ans.     What is the number of H+ ions in 98 amu ( 3 + 31 + 64 = 98). 9.8 g of H3PO4 equals 0.1mole. 1 molecule of H3PO4 contains 3H+ ions. And 0.1 mole of H3PO4 has 0.3 moles of H+ ions. Thus 0.3 moles of H+ ions contains 0.3 x 6.02 x 1023 = 1.806 x 1023 H+ ions.

Q.63.   calculate the mass of 3.01 x 1023 molecules of CO2.

Ans.     6.02 x 1023 molecules of CO2 = 1 mole of CO2 = 44g CO2

3.01 x 1023x 1023 molecules of CO2 have mass = mass of CO2 / molar mass of CO2 X NA (6.02 x 1023)

=22 g

Q.64.   How many carbon atoms are in 18g of carbon?

Ans.     Atomic mass of carbon = 12 amu

No. of carbon atoms =  x NA

x 6.02cx 1023 C atoms = 9.03 x 1023 C atoms

Q.65.   How many molecules of water are in 24 g of ice?

Ans.     Molar mass of water (ice) = 18 g mol-1

No. of H2O molecules =         x NA

x 6.02 x 1023 = 9.03 x 1023 molecule

Q.66.   How many isotopes of hydrogen are known? Write their names and symbols:

Ans.     Three isotopes of hydrogen are known. Their names and symbols are protium, deuterium, and tritium,

Q.67.   How many isotopes of carbon are known? How are they written and expressed?

Ans.     Carbon has three isotopes written as  and expressed as C-12, C-13 and C-14. Each of these have six protons and 6-electrons.

Q.68.   what are the relative atomic masses of hydrogen, oxygen, Ne and chlorine?

Ans.     The relative atomic mass, of hydrogen is 1.008 amu, of oxygen 15.9994 amu, of Ne is 20.171797 amu and of Cl is 35.546 amu.

Q.69.   what unit is used to expressed the atomic mass?

Ans.     The unit used to express the relative atomic mass is called atomic mass unit (amu) and it is 1/12th of the mass of carbon.

Q.70.   what is mathematical relationship for m/e?

Ans.     The mathematical relationship for m/e is

m/e = H2r2/2E

where H is the strength of magnetic field, E is the strength of electric field, r is the radius of circular path. If E is increased, by keeping H constant then radius will increase and a particular ion falls at different place as compared to first one.

Q.71.   Which equation is used to determine the percentage composition  of a compound?

Ans.     The following equation is used

Percentage of an element =  x 100

Q.72.   Differentiate between mole and Avogadro’s number?

Ans.     The atomic mass, molecular mass or formula unit  mass of a substance expressed in grams is called a mole. While the number of atoms, molecules or ions in one mole of a substance is called Avogadro’s.

Q.73.   which type of relationship can be studied wit the help of a balanced equation for stoichiometric calculations?

Ans.     The following type of relationship can be studied.

  1. Mass-mass relationship            2. Mass-mole relationship        3. Mass-volume relationship 4. Mole-mole calculations can also be performed.

Q.74.   which are the three steps to identify a limiting reactant?

Ans.     1. Determine the  number of moles from the given amount of reactant.

            2. Find out the number of moles of product with the help of a balanced chemical equation.

            3. Identify the reactant which produces the least amount of product as limiting reactant.

Q.75.   How will you calculate the average atomic mass from relative abundance of isotopes?

Ans.     Multiply atomic mass of each isotope of an element by its relative abundance, then add them and divide by 100. This gives the average that element.

Q.76.   many chemical reactions taking place in our surrounding involve the limiting reactants?

Ans.     In our surrounding, many combustion reactions take place in which burning substance is a limiting reactant and oxygen in excess, e.g., petroleum burns in excess of oxygen present in air, burning of natural gas, burning of carbon.

Nitrogen and oxygen of the atmosphere reacts to form NO due to electrical discharges of lightening. In this reaction oxygen is limiting reactant and nitrogen in excess. This reaction show that many chemical reaction taking place in our surrounding involve the limiting reactant.

Q.77.   Give reason that law of conservation of mass has to be obeyed during stoichiometric calculation?

Ans.     It is assumed in stoichiometric calculation that atoms are neither created nor destroyed during a chemical change. Thus, the total mass of the products of a chemical reactions is the same as the total mass of the reactants and this statement is known as law of conservation of mass. The changes that occur during any reaction merely involve rearrangement of atoms. These arguments support that law of conservation of mass has to be obeyed during stoichiometric calculations.

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