Back titration exercise 2

1.973 g of impure potassium nitrate was reacted with 2 g of aluminium (an excess) and 25 cm³ of 2 M sodium hydroxide solution (an excess) according to the following equation:

3NO₃^-(aq) + 8Al(s) + 5OH^-(aq) + 2H₂O(l) ⇒ 8AlO₂^-(aq) + 3NH₃(g)

The reaction was carried out in a sealed container with the evolved ammonia gas being dissolved in 25.0 cm³ of 1.000 M hydrochloric acid through an inverted funnel. The original reaction mixture was briefly boiled. The excess hydrochloric acid was then titrated with 0.500 M sodium hydroxide solution. The titre was 12.60 cm³.

What is the 25 cm³ of 2 M sodium hydroxide solution measured out with?

Why was the original reaction mixture briefly boiled?

What is the percentage purity of the potassium hydroxide?

Solution to exercise