Numerical Problems : Stoichiometry

Level – 1

1. A definite amount of NH4Cl was boiled with 100 mL of 0.8 N NaOH for complete reaction of NH4Cl. After the reaction, the reactant mixture containing excess of NaOH was neutralized with 12.5 mL of 0.75 NH2SO4. Calculate the amount of NH4Cl taken.

2. In a quality control analysis for sulphur impurity 5.6 gm steel sample was burnt in a stream of oxygen and sulphur was converted to sulphate by using H2O2 solution to which had been added 30 mL of 0.04 M NaOH. The equation for reaction is

SO2(g) + H2O2(g) + 2OH(aq) –> SO42- + 2H2O(l)

22.48 ml of 0.624 M HCl was required to neutralize the base remaining after oxidation reaction. Calculate % of sulphur in given sample.

3. 2.480 gm KClO3 are dissolved in conc. HCl solution and chlorine gas evolved in the reaction was then passed through a solution of KI and liberated iodine was treated with 100 mL of hypo solution 12.3 mL of same hypo solution required 24.6 mL of 0.5 N iodine for complete neutralization. Calcualte % purity of KClO3 sample.

4. To a 25 ml H2O2 solution, excess of acidified solution of KI was added. The iodine liberated required 20 ml 0.3 N Na2S2O3 solution. Calculate the volume strength of H2O2.

5. 1 gm sample of KClO3 was heated under such conditions that a part of it decomposed according to the equation
i) 2KClO3 –> 2KCl + 3O2
ii) and the remaining underwent change according to the equation
4KClO3 –> 3KClO4 + KCl
If the amount of O2 evolved was 146.8 ml at STP, calculate percentage by weight of KClO4 in the residue.

6. A solid mixture (5 gm) 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 lead nitrate and sodium nitrate in the mixture.

7. A 8.0 g sample contained Fe3O4, Fe2O3 and inert materials. It was treated with an excess of aqueous KI solution in acidic medium, which reduced all the iron to Fe+2ions.
The resulting solution was diluted to 50.0 cm3 and a 10.0 cm3 of it was taken. The liberated iodine in this solution required 7.2 cm3 of 1.0 M Na2S2O3 for reduction to iodide.
The iodine from another 25.0 cm3 sample was extracted, after which the Fe+2 ions was titrated against 1.0 M KMnO4 in acidic medium. The volume of KMnO4 solution used was found to be 4.2 cm3. Calculate the mass percentages of Fe3O4 and of Fe2O3 in the original mixture.

8. H2O2 is reduced rapidly by Sn2+ , the products being Sn4+ and water. H2O2 decomposes slowly at room temperature to yield O2 and water. Calculate the volume of O2 produced at 20oC and 1 atm. when 200 gm. of 10% by mass H2O2 in water is treated with 100 ml. of 2 M Sn2+ and then the mixture is allowed to stand until no further reaction occurs.

9. Calculate the % of MnO2 in a sample of pyrolusite ore, 1.5 g which was made to react with 10 g of Mohr’s salt (FeSO4 .(NH4 )2 SO4 . 6H2 O) and dilute H2 SO4 . MnO2 was converted 2 Mn2+ . After the reaction the solution was diluted to 250 ml and 50 ml of this solution, when titrated with 0.1 N K2 Cr2 O7, required 10 ml of the dichromate solution.

10. A 1.2 g. of a mixture containing H2 C2 O4 .2H2 O and KHC2 O4 .H2 O and impurities of a neutral salt, consumed 18.9 ml of 0.5 N NaOH for complete neutralization. On titiration with KMnO4 solution 0.4 g of the same substance needed 21.55 ml of 0.25 N KMnO4 . Calculate the percentage composition of the substance.

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