Core practical 3 - find the concentration of a solution of hydrochloric acid

Write a chemical equation for the reaction of hydrochloric acid with sodium hydroxide

HCl + NaOH → NaCl + H₂O

The pink colour seen at the end point fades because the excess sodium hydroxide reacts with carbon dioxide in the air. This reaction makes sodium carbonate.

(a) Write an equation for the reaction of sodium hydroxide with carbon dioxide.

(b) When sodium hydroxide solution is stored, it reacts with carbon dioxide in the air. (i) How will this change the concentration of the sodium hydroxide solution?

(ii) How will this affect the volume of sodium hydroxide solution required to reach the end point in the titration? Explain your answer.

(a) 2NaOH + CO₂ → Na₂CO₃ + H₂O

(b)(i) Decrease

(ii) The volume of sodium hydroxide solution will not change because the sodium carbonate made will also react with the hydrochloric acid. Two moles of sodium carbonate react to make one mole of sodium hydroxide which will react with two moles of hydrochloric acid, the same number of moles of hydrochloric acid that the sodium hydroxide would have reacted with.

Explain why it is better to have a titre of around 25 cm³ than of around 10 cm³.

The percentage error will be larger, the smaller the titre

Safety

• Wear eye protection

• Phenolpthalein solution is flammable and toxic

• Read burette from eye level - bottom of meniscus