Can you solve them now

“Gain confidence”

NUMERICAL PROBLEMS OF FULL CHAPTER

[Atomic mass: H =1. C =12. N =14. O = 16, S = 32, Mg = 24, Na = 23, Na, Al = 27,
K = 39, Ca = 40, Au = 197]
SIGNIFICANT FIGURES AND CONVERSIONS
1. State the number of significant figures in each of the numbers: [NCERT]
4
(i) 2.653 × 10 (ii) 0.003688 (iii) 653 (iv) 0.368 (v) 0.0300
[(i) 4, (ii) 4, (iii) 3, (iv) 3, (v) 3]
2. Express the following numbers to four significant figures [NCERT]
3
(i) 5.607892 (ii) 32.392800 (iii) 1.78986 × 10 . (iv) 0.007837
[(i) 5.608, (ii) 32.39, (iii) 1.790 × 10 3 (iv) 7.837 × 10-3]
3. Express the result of the following calculations to the appropriate number of significant figures.
[NCERT]
3.24  0.08666
(i) (ii) 0.58 + 324.65 (iii) 943 × 0.00345 + 101
5.006
[(i) 5.60 × 102, (ii) 325.23 (iii) 104.25]
4. Convert the following in kilogram. [NCERT]
5
(i) 500 Mg [mass of jumbo jet (loaded)] [5.0 × 10 kg]
-24
(ii) 3.44 × 10 g (mass of hydrogen molecule) [3.34 × 10-27 kg]
5. Convert into metre [NCERT]
-9
(i) 7 Å (diameter of small virus) [7 × 10 m]
(ii) 1.4 Gm (diameter of Sun) [1.4 × 109m]
6. Convert 1 amu into MeV. [931.44 MeV]
7. For a precious stone ‘carat’ is used for specifying its mass. If 1 carat = 3.168 grains (a unit of
mass) and 1 gram = 15.4 grains, find the total mass in kilogram of the ring that contains 0.5 carat
diamond and 7 gram gold. [7.103 × 10-3 kg]

LAWS OF CHEMICAL COMBINATION

1. Two elements X and Y form two different compounds. In the first compound, 1g of X combined
with 1.44 g of Y. In the second, 0.15 g of X combined with 0.65 g of Y. Verify the law of multiple
proportions.
2. (a) 0.46 g of magnesium produces 0.77 g of magnesium oxide,
(b) 0.82 g of magnesium liberated 760 ml of hydrogen at NTP from an acid (wt. of
1 ml of hydrogen at NTP = 0.00009 g) and
(c) 1.25 g of water from the union of 1.11 g of oxygen with hydrogen.
Show that these results illustrate the law of reciprocal proportions.

MOLE CONCEPT
1. Calculate the mass in grams of (i) 0.25 mole ammonia (ii)20 moles of H2SO4
[(i) 4.25 g (ii) 1960 g]
2. How many moles are present in. (i) 12.8 g SO2? (ii) 4.0 g NaOH?
[(i) 0.2 mol (ii) 0.1 mol]
3. Calculate the number of molecules present. (i) One drop of water weighing 0.05 g
(ii) 250 mL of water (Density of water is 1 g mL -1) [(i) 1.6722 × 10-21 (ii) 8.3638 × 1024]
4. Which of the following has maximum number of atoms?
(i) 6g C (ii) 1g H2 (iii) 12g Mg (iv) 30g Ca [(ii) 1g H2]
5. 19.7 kg gold was recovered from a smuggler. How many atoms of gold were recovered?
[(6.02 × 1025]
6. The cost of table salt (NaCl) is Rs. 2 per kg. Calculate its cost per mole?
[Rs. 0.117 or 12 paise]
24
7. Calculate the mass of 6.00 × 10 molecules of sulphur dioxide? [640 g]
8. Calculate the number of moles of atoms present in one atom of an element?
[1-66 × 10-24]
9. One atom of an element weighs 6.644 × 1 0 -23 g. Calculate the number of g atoms in 40 kg of it.
(6.02 × 1026 atoms; 1000 g atom)
10. Calculate the number of moles of SO2 containing 6.4 g oxygen? [0.2 mol]
11. Calculate the number of atoms in each of the following:
(i) 52 moles of He (ii) 52 amu of He (iii) 52 g He
((i) 52 NA (ii)13 (iii) 13 NA)
23 2-
12. Compute the number of moles of aluminium sulphate which contain 10 SO4 ions.
(5.54 × 10-2)
13. A gaseous mixture contains oxygen and nitrogen in the ratio of 1:4 by mass. Compute the ratio
of their number of molecules [7:32]
-26
14. The mass of water molecules is 2.99 × 10 kg. Calculate the number of water molecules
contained in one cubic metre of water. [3.34 × 1 028]
15. How many electrons are present in 18 ml of water? [6.02 × 1024]
16. How many years it would take to spend Avogadro’s number of rupees at the rate of
10 lac rupees per second? [1.9099 × 1010 years]
17. The density of O at NTP is 1.429 g/L. Calculate the standard molar volume of gas.
[22.39 litre mol-1]
18. An alloy has Fe, Co and Mo equal to 71%, 12% and 17% respectively. How many cobalt atoms
are there in a cylinder of radius 2.50 cm and a length of 10.0 cm. The density of alloy is 8.20
g/mL. Atomic weight of cobalt = 58.9. [19.8 × 1023]
19. Calculate the number of Cl- and Ca2+ ions in 222 g anhydrous CaCl2. [4 N ions of Cl-]

PERCENTAGE COMPOSITION, EMPIRICAL AND

MOLECULAR FORMULAE
1. Find the empirical formula of a substance whose composition includes hydrogen, carbon, oxygen
and nitrogen in the mass ratio of 1:3:4:7. [H4CON2]
2. Vitamin C (ascorbic acid) contains 40.92% C, 4.58% H and 54.50% O by mass. What is the
empirical formula of ascorbic acid? If molecular weight of ascorbic acid is
176 g mol-1, what is the molecular formula? [C3H4O3, C6H8O6]
3. The empirical formula of a gaseous compound of carbon with hydrogen is CH 2. What is the
molecular formula of the compound if the mass of one litre of the compound equals that of one
litre of nitrogen at N.T.P. [C2H4]
4. A crystalline salt on being rendered anhydrous loses 45.6% of its mass. The percentage
composition of the anhydrous salt is Al = 10.46%, K = 15.11%,
S = 24.80%, O = 49.61%. The formula mass of the salt is 518 amu. Determine the formula of the
anhydrous and crystalline salt. [K2Al2S4O16 ; K2SO4Al2 (SO4)3. 24 H2O]
5. A compound of carbon, hydrogen and nitrogen contains three elements in the respective ratio of
9:1 : 3.5. If the molecular mass of the compound is 108 amu, calculate the molecular formula of
the compound. [C6H8N2]
CHEMICAL EQUATIONS
1. Calculate the number of moles of sodium chloride needed to react with excess of silver nitrate to
produce 28.7 g silver chloride. [0, 2]
2 Compute the amount of sulphur precipitated by bubbling 112 ml of H 2S gas through excess of
ferric chloride solution at N.T.P. [0.16 g]
2FeCl3 + H2S → 2FeCl2 + 2HCl + S
3. 40 g of a mixture of calcium carbonate and sand is treated with an excess of hydrochloric acid
and 8.8 g CO2 is produced. Compute the percentage of calcium carbonate in the mixture.
[50%]
4. Calculate the number of moles of water formed by igniting 200 g hydrogen and 200 g oxygen.
[12.5 mol]
5. 4.6 g of sodium is treated with excess of water. Compute the volume of hydrogen gas evolved at
S.T.P. [2240 ml]
6. Calculate the mass of CaO required to remove the hardness of 10 5 L of water containing 1.62 g
of calcium bicarbonate per litre. [56 kg]
7. How many litres of oxygen at S.T.P. will be required to burn completely 2.2 g of propane?
[5.6 L]
MISCELLANEOUS PROBLEMS
1. Convert 22400 cm3 into cubic meters. [2.24 × 10-2m3]
2. The density of benzene at 25°C is 0.879 g cm -3. What volume, in litres, will be occupied by 2.50
kg of benzene? [2.84 L]
3. Perform the calculation indicated and express the answer to the correct number of significant
digits. Use exponential notation where needed
(1) 17.2 × 12.55 (ii) 1.4 × 1.11/42.33
(iii) 25.7 -25.25 (iv) 2.00 × 103 - 1.7 × 101
(i) 2.16 × 102 (ii) 3.7 × 10-2 (iii) 0.4 (iv) 1.98 × 103
4. A naturally occurring compound C22H25NO6 has been used as a medicine since ancient times :
(i) What is the molar mass of this compound?
(ii) What is the mass, in grams, of 3.2 × 10 22 molecules of the compound?
(iii) How many moles are there in 326 g of the compound?
(iv) How many carbon atoms are in 50 molecules of the compound?
(i) 399.0 g mol-1 (ii) 21.2 g (iii) 0.817 mol (iv) 1.1 × 103 atoms
5. What is the mass percent of carbon in CO? What is the mass of carbon in 4.9 g CO?
[42.9%; 2.1 g]
6. What is the mass of a mole of water containing 50% of heavy water? [19 g]
7. Compute the number of molecules in each of the following:
(i) 400 moles of NaOH (ii) 400 amu of NaOH
(iii) 400 g of NaOH [(i) 2.40 × 1028 (ii) 10 (iii) 6.02 × 1024]
8. Find (i) the total number of neutrons and (ii) the total mass of neutrons in 7 mg of 14C (Assume
that the mass of a neutron = mass of a hydrogen atom).
(i) 2.408 × 1021 (ii) 4.032 × 10-3 g)
9. Calculate the following with respect to (NH4)2SO4
(i) Number of moles of (NH4)2SO4 which contain 280 g of nitrogen
 (ii) Number of formula units of (NH4)2 SO4 which contain 0.2 motes of oxygen atoms.
(iii) Number of grams of (NH4)2SO4 which contain 1 × 1023 SO42- ions.
((i) 10 (ii) 3.01 × 1022 (iii) 21.92 g)
10. The density of water at 4°C is 1.0 g cm3. What is the volume occupied by 1 mole of water at 4°C?
[18 cm3]
11. Two identical flasks contain equal masses of CO 2 and N2O respectively. Do they contain equal
number of molecules or not? Why? [Yes]
22
12. From 6 × 10 molecules of N2 present in a container, 700 mg are removed. How many molecules
are left behind? How many grams of H2 would be needed to convert left over N2 into NH3? (NA =
6 × 1023 mol-1). [4.5×1022; 0.45 g]
13. A solid mixture (5.0 g) consisting of lead nitrate and sodium nitrate was heated below 600°C until
the weight of the residue was constant. If the loss in weight is 28 percent, find the amount of lead
nitrate and sodium nitrate in the mixture.
2 Pb (NO3)2 → 2 PbO + 4NO  + O2  ; 2 NaNO3 → 2NaNO2 + O2?
[3.325 g ; 1.675 g]
14. Gastric juice contains 3 g HCl per litre. If a person produces 2.5 L of gastric juice per day, how
many antacid tablets each containing 400 mg of Al (OH) 3 are needed to neutralize all the HCl
produced in one day? [Al (OH) 3 + 3HCl → AlCl3 + 3H2O]. [14 tablets]
3
15. 250 cm of 0.5 M Na2SO4 solution is added to an aqueous solution of 15.0 g BaCl 2 to form BaSO4
(a white ppt). How many moles and how many grams of BaSO4 are formed? (BaCl2 + Na2SO4 →
BaSO4 + 2 NaCl). [0.072 mol; 16.79 g]
22
16. A sample of compound contains 2.04 g sodium, 2.65 × 10 atoms of C and 0.132 mol of
oxygen atoms. Compute empirical formula of the compound. [Na2CO3]
17. What is the formula of a compound whose per mole contains 12 gram - atoms of carbon, 1.3244
× 1025 atoms of hydrogen and 176 gram oxygen. [C12H22O11]
18. Two oxides of a metal contain 27.6 and 30.0% of oxygen respectively. If the formula for first oxide
be M3O4, find that of the second? [M2O3]
19. Calculate the concentration of nitric acid in moles per litre in a sample which has a density 1.41
g mL-1 and the mass percent of nitric acid in it being 69%. [7.77 M]
-1
20. If the density of methano1 is 0.793 kg L , what is its volume needed for making
2.5 L of its 0:25 M solution? [NCERT] [0.12]
21. Calculate the molarity of a solution of ethanol in which the mole fraction of ethanol is 0.040
(assume the density of water to be one). [NCERT] [2.31 M]
22. The average atomic mass of copper is 63.5. It exists as two isotopes which are 29Cu63 and 29Cu65.
Calculate the percentage of each isotope present in it [75:25]