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STOICHIOMETRY 1. How many litres of detonating gas will be produced at NTP in thedecomposition of 0.1 mole of water

by an electric current ? Ans. 3.36 litres 2. When the mixture of MgCO3 and CaCO3 was heated for a long time. The weight decreased by 50%.

Calculate the percentage composition of the mixture. Ans.MgCO3 : 7159% ; CaCO3 : 28.41%

3. How many moles of Zn(FeS2)2 can can be made from 2 g of zn, 3g of Fe and 4 g of S ?Ans0.0269

mole 4. Equal weights of Zn metal and iodine are mixed together and the iodine is completely converted to

ZnI2. What fraction by weight of the original zinc remains unreacted? Ans. 0.74. 5. 1.0 g of an alloy of Al and Mg when treated with excess of dil. HCl forms MgCl2, AlCl3 and hydrogen.

Collected over Hg at 0°C, has a volume of 1.20 litres at 0.92 atm pressure. Calculate the composition of the alloy. (al = 27 and Mg = 24). Ans. 45%

6. A 1g mixture of cuprous oxide and cupric oxide was quantitavely reduced to 0.839 g of metallic

copper. What was the weight of cupric oxide in the original sample ? Ans.0.55g 7. 27.6 g of K2CO3 was treated by a series of reagents so as to convert all of its carbon to K2 -

Zn3[Fe(CN)6]2. Calculate the weight of the product. Ans. 11.6 g 8. In a gravimetric determination of P, an aqueous solution of dihydrogen phsosphate ion H2PO4

� is treated with a mixture of ammonium and magnesium ions to precipitte magnesium ammonium phosphate, Mg(NH4)PO4.6H2O. This is heated and decomposed to magnesium pyrophosphate, Mg2P2O7, which is weighed. A solution of H2PO4

� yielded 1.054 g of Mg2P2O7. What weight of NaH2PO4 was present originally ? Ans. 1.14 g.

9. Equal weights of phosphorus and oxygen are heated in a closed vessel producing P2O3 and P2O5 in

a 1 : 1 mole ratio. If the limiting component is exhausted , find which component and also what fraction of it is left over. Ans. 3.125%

10. 10 cc of H2O2 solution when reacted with Ki solution produced 0.5 g of iodine. Calculate the

percentage purity of H2O2. (I = 127) Ans. 0.669% 11. 0.32 mole of LiAlH4 in ether solution was placed in a flask and 74 g (1mole) of t-butyl alcohol was

added. The product LiAlHC12H27O3 weighed 81.28 g. Is the claim that the yield is 'quantitative' a valid one if Li atoms are conserved ? (Li = 7m, Al = 27, H = 1, C = 12, O = 16)Ans. yield is 100%

12. 52.5 millimoles of LiAlH4 was treated with 15.6 g (210 millimiles) of t-butyl alcohol. A total of 157.5

millimoles of hydrogen was evolved for the reaction. LiAlH4 + 3(CH3)3COH H2 + Li(CH3)3O]3[CH3O]Al The addition of an excess of another alcohol, methanol, to the above reaction mixture caused the fourth H atom of the LiAlH4 to be replaced according to the equation. Li(CH3)3O]3 AlH + CH3OH H2 + Li[(CH3)3O]3[CH3O] Al How much H2 was evolved due to the addition of CH3OH ? Ans. 52.5 millimoles 13. What weight of CO is required to form Re2(CO)10 from 2.50 g of Re2O7 according to the unbalanced

reaction : Re2O7 + CO Re2(CO)10 + CO2 (Re = 186.2, C = 12 and O = 16 Ans. 2.46 g

14. 1-g sample of KClO3 was heated under such conditions that a part of it decomposed according to the

eqaution : (i) 2KClO3 = 2KCl + 3O2 and the remaining underwent change according to the equation (ii) 4KClO3 = 3KClO4 + KCl If the amount of O2 evolved was 146.8 mL at NTP, calculate the percentage by weight of KClO4 in the residue. Ans. 49.85%

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MASTERING PHYSICAL CHEMISTRY By-- S.K.SINHA

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15. 1g of dry green algae absorbs 4.7 × 10�3 mole of CO2 per hour by photosynthesis. If the fixed carbon atoms were all stored after photosynthesis as starch, (C6H10O5)n, how long would it take for the algae to double their own weight assuming photosynthesis takes place at a constant rate ? Ans. 7.88 hours

16. A mixture of NaI and NaCl, when heated with H2SO4, produced the same weight of sodium sulphate

as that of the original mixture. Calculate percentage of NaI in the mixture.Ans. 28.85% 17. A mixture of KBr and NaBr weighing 0.560 g was treated with aqueous Ag+ and all the bromide ion

was recovered as 0.970 g of pure AgBr. What was the fraction by weight of KBr in the sample ? (K = 39, Br = 80, Ag = 108, Na = 23 Ans. 0.2378 g

18. 1.84 g of a mixture of CaCO3 and MgCO3 was heated to a constant weight. The constant weight of

the residue was found to be 0.96 g. Calculate the percentage composition of the mixture. (Ca = 40, Mg =24, C = 12, O =16) Ans. 45.66%

19. A sample of mixture of CaCl2 and NaCl weighing 4.22 g was treated to precipitate all the Ca as

CaCO3, which was then heated and quantitatively converted to 0.959 g of CaO. Calculate the percentage of CaCl2 in the mixture. (Ca = 40, O = 16, C = 12 and Cl = 35.5) Ans. 45.04%

20. A 2-g sample containing Na2CO3 and NaHCO3 loses 0.248 g when heated to 300°C, the

temperature at which NaHCO3 decomposes to Na2CO3, CO2 and H2O. What is the percentage of Na2CO3 in the given mixture? (Na = 23, C = 12, O = 16 and H = 1) Ans.66.4%

21. A solution of lead nitrate prepared by dissolving 2.07 g of pure lead in nitric acid was treated with

HCl, Cl2 gas and NH4Cl. What will be the maximum weight of (NH4)2 PbCl6 so produced ? (N = 14, H = 1, Pb = 207, Cl = 35.5) Ans. 50%

22. Equal weights of mercury and iodine are allowed to react completely to form a mixture of mercurous

and mercuric iodide leaving one of the reactants. Calculate the ratio by weight of Hg2I2 and HgI2 formed. Ans. 0.532 : 1

23. 1g of a sample containing NaCl, NaBr and an inert material, with excess of AgNO3, produces 0.526 g

of precipitate of AgCl and AgBr. By heating his precipitate in a current of chlorine. AgBr converted to AgCl and the precipitate then weighed 0.426 g. Find the percentage of NaCl and NaBr in the sample. ans. NaCl : 4.25%, NaBr : 23.2%

24. One litre of an acidified solution of KMnO4 containing 15.8 g of KMnO4 is decolourised by passing

sufficient amount of SO2. If SO2 is produced by roasting iof iron pyrites (FeS2). What will be the amount of pyrites required to produce the necessary amount of SO2 ? Ans. (15 g)

25. When a mixture of NaBr and NaCl is repeatedly digested with sulphuric acid, all the halogens are

expelled and Na2SO4 is formed quantitatively. With a particular mixture, it was found that the weight of Na2SO4 obtained was precisely the same as the weight of NaBr-NaCl mixture taken. Calculate the ratio of the weights of NaCl and NaBr in the mixture. Ans. 1.454 : 1

26. 25.4 g of iodine and 14.2 g of chlorine are made to react completely to yield a mixture jof Icl and ICl3

Calculate the number of moles of ICl and ICl3 formed. Ans. 0.1 mole, 0.1 mole 27. Three different brands of 'liquid chlorine' are available in the market for use in purifying water of

swimming pools. All are sold at the same rate of Rs. 10 per litre and all are water solutions. Brand A contains 10% hypochlorite (ClO) (wt./vol.), brand B contains 7% available chlorine (Cl) and brand C contains 14% sodium hypochlorite (NaClO). Which of the three would you buy ?Ans. 6.67 g

28. In the analysis of a 0.50 g sample of feldspar, a mixture of the chlorides of Na and K is obtained,

which weighs 0.1180g. Subsequent treatement of the mixed chlorides with silver nitrate gives 0.2451 g of AgCl. What is the percentage of sodium oxide and potassium oxide in feldspar ? Ans. Na2O =3.6% , K2O = 10.6%

29. A mixture contains NaCl and an unknown chloride MCl.

(i) 1g of this is dissolved in water. Excess of acidified AgNO3 solution is added to it. 2.567 g of a

MASTERING PHYSICAL CHEMISTRY By-- S.K.SINHA

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white precipitate is formed. (ii) 1.0 g of the original mixture is heated to 300°C. some vapours come out which are absorbed in acidified AgNO3 solution. 1.341 g of a white precipitate is obtained. Find the molecular weight of the unknown chloride. Ans. 53.5

30. A solid mixture (5g) consisting of lead nitrate and sodium nitrate was heated below 600°C until the

weight of the residue is constant. If the loss in weight is 28%, find the amount of the lead nitrate and sodium nitrate in the mixture. Ans. 1.6754 g.

31. In a particular experiment, 272 g of phosphorus, P4, reacted with excess of oxygen to form P4O10 in

89.5% yield. In the second step of the reaction, a 97.8% yield of H3PO4 was obtained. What mass of H3PO4 was obtained. Ans. 752.65 g

32. What weights of P4O6 and P4O10 will be produced by the combustion of 2g of P4 in 2g of oxygen

leaving no P4 and O2 Ans. 1.996 g, 2.004

33. For the production of equal amounts of hydrogen from the following reactions, which metal, Zn or Al, is less expensive if Zn costs about half as much as Al on a mass basis and by how much? Zn + 2HCl ZnCl2 + H2 2Al + 6HCl 2AlCl3 + 3H2 Ans. 44.61%

34. A mixture of pure AgCl and pure AgBr is found to contain 60.94% Ag by mass. What are mass

percentages of Cl and Br in the mixture ? (Ag = 108, Cl = 35.5, Br = 80)Ans. Cl = 4.85%, Br = 34.19%

35. From the following reaction sequence,

CaC2 + H2O CaO + C2H2 C2H2 + H2 C2H4 nC2H4 (C2H4)n Calculate the mass of polyetheylene which can be produced from 10 kg of CaC2 Ans. 4375g

36. From the following series of reactions,

Cl2 + 2KOH KCl + KClO + H2O 3KClO 2KCl + KClO3 4KClO3 3KClO4 + KCl Calculate the mass of chlorine needed to produce 100 g of KClO4. Ans. 204.5 g

37. Crude calcium carbide is made in an electric furnace by the following reaction : CaO + 3C CaC2 + CO The product contains 85% of CaC2 and 15% of unreacted CaO. (a) How much CaO is to be added to the furnace charge for each 1000 kg of CaC2 (pure) produced? (b) How much CaO is to be added to the furnace charge for each 1000 kg of crude product ? Ans. (a) 1041.5 kg ; (b) 893.7 kg

38. From the following reactions, 2CoF2 + F2 2CoF3 (CH2)n + 4n CoF3 (CF2)n + 2nHF + 4n CoF2 calculate how much F2 will be consumed to produce 1 kg of (CF2)n. Ans. 1.52 kg

39. A mixture containing KClO3, KHCO3, K2CO3 and KCl was heated, producing CO2, O2 and H2O gases

according to the following equations : 2KClO3(s) 2KCl(s) + 3O2 2KHCO3(s) K2O(s) + H2O(g) + 2CO2(g) K2CO3(s) K2O(s) + CO2(g) The KCl does not react under the conditions of the reaction. If 100.0 g of the mixture produces 1.80 g of H2O, 13.20 g of CO2 and 4.0 g of O2, what was the composition of the original mixture ? Ans. (KClO3 : 10.2 g, KHCO3 : 20g, K2CO3 : 13.8 g

40. A natural gas sample contains 84% (by volume) of CH4, 10% of of C2H6 , 3% of C3H8 and 3% of N2. If

a series of catalytic reactions could be used for converting all the carbon atoms of the gas into butadiene, C4H6, with 100% efficiency, how much butadiene could be prepared from 100 g of the natural gas ? Ans. 82 g