A student added 3.5 g3.5\text{ g}3.5 g of zinc powder to hydrochloric acid. She observed that no zinc was left when the reaction was complete. The student transferred the solution to an evaporating basin. She heated the solution using a Bunsen burner and evaporated all the water.
Explain how you can tell from the student's observation that the hydrochloric acid was in excess.
Look at the equation for the reaction:
Zn(s)+2HCl(aq)→ZnCl2(aq)+H2(g) \text{Zn(s)} + 2\text{HCl(aq)} \rightarrow \text{ZnCl}_2\text{(aq)} + \text{H}_2\text{(g)} Zn(s)+2HCl(aq)→ZnCl2(aq)+H2(g)The student knows the reaction is complete when there is no zinc left. Use the equation to explain one other way the student could tell that the reaction was complete.
The student predicts she should make 11.4 g11.4\text{ g}11.4 g of zinc chloride, ZnCl2\text{ZnCl}_2ZnCl2. She actually makes 9.3 g9.3\text{ g}9.3 g. Calculate the percentage yield. Give your answer to 3 significant figures.
Write down one reason, other than a mistake, why the student may have obtained a percentage yield of less than 100%100\%100%.
91 exam-style questions on OCR GCSE Chemistry Monitoring chemical reactions. Each one has a worked solution and a mark scheme showing where the marks go.