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How Much? The Amount of Chemical Change · Paper 2 — Free Chemistry SL Practice Questions

1ChallengeSAQ-LStoichiometry and mole-to-mole ratios7 marksPaper 2~11 min

A student investigates the reaction between sodium carbonate (

\text{Na}_2\text{CO}_3

) and hydrochloric acid (HCl):

\text{Na}_2\text{CO}_3(\text{s}) + 2\,\text{HCl}(\text{aq}) \rightarrow 2\,\text{NaCl}(\text{aq}) + \text{CO}_2(\text{g}) + \text{H}_2\text{O}(\text{l})

The student adds

2.65\,\text{g}

of anhydrous sodium carbonate to a flask containing

50.0\,\text{cm}^3

1.00\,\text{mol\,dm}^{-3}

hydrochloric acid. After the reaction, the remaining acid is titrated with

0.200\,\text{mol\,dm}^{-3}

sodium hydroxide solution, using phenolphthalein indicator. The titration requires

15.0\,\text{cm}^3

of sodium hydroxide solution to reach the endpoint. - Molar mass of

\text{Na}_2\text{CO}_3 = 106.0\,\text{g\,mol}^{-1}

(a)

Calculate the number of moles of hydrochloric acid that remain unreacted after the reaction with sodium carbonate. [2 marks]

(b)

Explain why the student uses excess hydrochloric acid rather than the exact stoichiometric amount. [2 marks]

(c)

Evaluate whether the experimental data is consistent with the

2:1

mole ratio of HCl to

\text{Na}_2\text{CO}_3

predicted by the balanced equation. Support your answer with calculations. [3 marks]

Solutions

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