A researcher reacted 2.4 g2.4\text{ g}2.4 g of magnesium ribbon with dilute hydrochloric acid in a conical flask. She observed that no magnesium ribbon was left when the reaction was complete. The researcher then transferred the solution to an evaporating basin, heated it using a Bunsen burner, and evaporated all the water to obtain dry magnesium chloride crystals.
Explain how you can tell from the researcher's observation that the hydrochloric acid was in excess.
Look at the equation for the reaction: Mg(s)+2HCl(aq)→MgCl2(aq)+H2(g)\text{Mg(s)} + 2\text{HCl(aq)} \rightarrow \text{MgCl}_2\text{(aq)} + \text{H}_2\text{(g)}Mg(s)+2HCl(aq)→MgCl2(aq)+H2(g) The researcher knows the reaction is complete when there is no magnesium ribbon left. Use the equation to explain one other way the researcher could tell that the reaction was complete.
The researcher predicts she should make 9.50 g9.50\text{ g}9.50 g of magnesium chloride, MgCl2\text{MgCl}_2MgCl2. She actually makes 7.85 g7.85\text{ g}7.85 g. Calculate the percentage yield. Give your answer to 3 significant figures.
Write down one reason, other than a mistake, why the researcher may have obtained a percentage yield of less than 100%100\%100%.