ST JOHN´S COLLEGE JUNIOR COLLEGE
CHEMICAL CALCULATIONS
CHE 123-1,5
1. Potassium persulphate is used in photography to remove the last traces of hypo from
photographic papers and plates. A 0.8162 g sample was found to contain 0.2361 g of
potassium and 0.1936 g of sulphur. The rest was oxygen. The formula weight of this
compound was measured to be 270. What are the empirical and molecular formulae of
potassium persulphate. [10]
2. Powdered aluminum and iron(III) oxide react with a large evolution of heat per mole
according to the following equation: [6]
Al + Fe2O3 → Al2O3 + Fe
In one experiment, 3.00 mol of aluminum was mixed with 1.25 mol of iron(III) oxide.
A) Which reactant, if either was the limiting reactant?
b) Calculate the theoretical yield in moles of iron.
3. 13.2 g of a sample of zinc sulphate, ZnSO4.xH2O, was strongly heated until no further
change in mass was recorded. On heating, all the water of crystallisation evaporated as
follows: ZnSO4.xH2O ZnSO4 + xH2O. Calculate the number of moles of water of
crystallisation in the zinc sulphate sample given that 7.4 g of solid remained after strong
heating. [8]
4. If 10.4 grams of hydrogen, H2, and 9.14 grams of oxygen, O2, are placed together in a
container and allowed to react according to the equation how many grams of water, H2O,
would be produced by the reaction? What are the limiting and excess reactants? How
many grams of the excess reactant remain when the reaction stops? How many grams of
the excess reactant were used up during the experiment? What is the percent yield of
water if the actual yield is 8.75 grams of water? [14]
5. Aspirin, C9H8O4, is synthesized by the reaction of salicylic acid, C7H6O3, with acetic
anhydride, C4H6O3. [6]
2C7H6O3 + C4H6O3  2 C9H8O4 + H2O
a. When 20.0 g of C7H6O3and 20.0g of C4H6O3 react, which is the limiting reagent?
b. What mass in grams of aspirin are formed?
6.
A research supervisor told a chemist to make 100 g of chlorobenzene from the reaction of
benzene with chlorine and to expect a yield no higher that 65%. What is the minimum
quantity of benzene that can give 100 g of chlorobenzene if the yield is 65%? The equation
for the reaction is: [6]
C6H6 + Cl2 -----------> C6H5Cl + HCl
benzene
chlorobenzene
7. A solution of sodium hydroxide contained 0.250 mol dm-3. Using phenolphthalein
indicator, titration of 25.0 cm3 of this solution required 22.5 cm3 of a hydrochloric
acid solution for complete neutralization.
(a) write the equation for the titration reaction. [1]
(b) what apparatus would you use to measure out (i) the sodium hydroxide solution? (ii) the
hydrochloric acid solution? [2]
(c) what would you rinse your apparatus out with before doing the titration ? [1]
(d) what is the indicator colour change at the end-point?[1]
(e) calculate the moles of sodium hydroxide neutralised. [2]
(f) calculate the moles of hydrochloric acid neutralised. [2]
(g) calculate the concentration of the hydrochloric acid in mol/dm3 (molarity). [2]
8. If it takes 50 mL of 0.5 M Ca(OH)2 solution to completely neutralize 125 mL of
sulfuric acid solution (H2SO4), what is the concentration of the H2SO4 solution? [6]
9. Lactic acid, a chemical responsible for muscle fatigue, is a monoprotic acid. When
0.578 g of lactic acid is titrated with 0.206 M NaOH, a volume of 31.11 mL of
NaOH is used. What is the molar mass of lactic acid? [4]
HA + NaOH -> NaA + H2O
10. An acid, H2X, reacts with sodium hydroxide as shown in the equation below.
H2X(aq) + 2NaOH(aq) →2Na+ (aq) + X2– (aq) + 2H2O(l)
A solution of this acid was prepared by dissolving 1.92 g of H2X in water and making the
volume up to 250 cm3 in a volumetric flask. A 25.0 cm3 sample of this solution required
21.70 cm3 of 0.150 mol dm–3 aqueous NaOH for complete reaction. (i) Calculate the
number of moles of NaOH in 21.70 cm3 of 0.150 mol dm–3 aqueous NaOH (ii) Calculate
the number of moles of H2X which reacted with this amount of NaOH. Hence, deduce
the number of moles of H2X in the 1.92 g sample. (iii) Calculate the relative molecular
mass, Mr, of H2X. [8]
11. In an experiment to make crystals of hydrated copper(II) nitrate, a sample of 5.60 g of
copper(II) oxide was added to 50 cm3 of 2.50 mol dm–3 nitric acid. The following
reaction occurred. CuO(s) + 2HNO3 (aq)
Cu(NO3) 2 (aq) + H2 O(l) (a) Calculate the
number of moles of each reactant present, and use this to show that the copper(II) oxide
was in excess. The molar mass of copper(II) oxide, CuO, is 79.5 g mol–1 (b) The
copper(II) nitrate solution was heated gently to concentrate it, and then left to crystallize.
The mass of hydrated copper(II) nitrate crystals, Cu(NO3 ) 2 .6H2 O, obtained was 12.52
g. Calculate the percentage yield. The molar mass of Cu(NO3 ) 2 .6H2 O is 295.6 g mol–1
[10]
12. A sample of sodium hydrogencarbonate was tested for purity using the following method.
0.400g of the solid was dissolved in 100 cm3 of water and titrated with 0.200 mol
dm3 hydrochloric acid using methyl orange indicator.
23.75 cm3 of acid was required for complete neutralisation. [Ar's: Na = 23, H = 1, C = 12,
O = 16]
(a) Write the equation for the titration reaction. [1]
(b) Calculate the moles of acid used in the titration and the moles of sodium hydrogencarbonate
titrated. [1]
(c) Calculate the mass of sodium hydrogen carbonate titrated and hence the purity of the sample.
[2]
(d) If 0.400g of another group 1 hydrogencarbonate in its pure state, was titrated with the same
acid and it took 20.00 cm3 to neutralise it, calculate ...
(i) moles of acid needed for neutralisation and moles of hydrogen carbonate titrated [1]
(ii) the formula mass of the hydrogencarbonate [1]
(iii) by working out the atomic mass of M, suggest the identity of M in the group 1
hydrogencarbonate formula MHCO3 [1]