Stoichiometry, Ionic Equilibria and Titrations for the Chemistry Olympiad — Practice

Question 1

4 g of H2 reacts with 16 g of O2. Which is limiting and how much water forms?

Accepted Answer

H2 is 2 mol and O2 is 0.5 mol. Oxygen is limiting and gives 1 mol of water, 18 g.

Question 2

Find the pH of 0.1 M acetic acid (Ka = 1.8e-5).

Accepted Answer

[H+] = root(1.8e-6) = 1.34e-3, so pH = 2.87.

Question 3

A litre holds 0.1 mol acetic acid and 0.1 mol acetate (pKa 4.76). Find the pH after adding 0.01 mol HCl.

Accepted Answer

Acetate 0.09 and acid 0.11, so pH = 4.76 + log(0.09/0.11) = 4.67.

Question 4

Find the solubility of AgCl (Ksp 1.8e-10) in water and in 0.1 M NaCl.

Accepted Answer

In water s = 1.34e-5 M. In 0.1 M NaCl s = 1.8e-10/0.1 = 1.8e-9 M.

Question 5

How much 0.0200 M KMnO4 is needed to oxidise 25.0 mL of 0.100 M Fe2+ in acid?

Accepted Answer

2.5 mmol Fe2+ needs 0.5 mmol MnO4- since each takes 5 Fe2+. Volume = 0.5/0.0200 = 25.0 mL.

Question 6

State the pH at the half-equivalence point of a weak acid titration and why.

Accepted Answer

pH = pKa. Half the acid has become its conjugate base, so [A-] equals [HA] and Henderson-Hasselbalch gives pH = pKa.

Question 7

Which indicator suits the titration of a weak acid with a strong base and why?

Accepted Answer

Phenolphthalein, because the equivalence point is basic and its colour change lies in the steep part of the curve above pH 7.

Question 8

How should a burette be prepared before use?

Accepted Answer

Rinse it with the titrant it will hold, then fill and remove air from the tip.

Practice — Stoichiometry, Ionic Equilibria and Titrations for the Chemistry Olympiad