A student wants to prepare carbon dioxide gas in the laboratory. Which combination of reagents and collection method is appropriate?
Show the answer and explanation
Correct answer
C. Calcium carbonate and dilute hydrochloric acid, collected by downward delivery (upward displacement of air).
Principle or equation
Carbon dioxide is commonly prepared by reacting a carbonate (e.g., calcium carbonate) with a dilute acid (e.g., hydrochloric acid). CO2 is denser than air, so it is collected by upward delivery (downward displacement of air). It is soluble in water, so not collected over water.
Why this answer is correct
CaCO3 + 2HCl → CaCl2 + H2O + CO2. CO2 has a molar mass of 44 g/mol, which is greater than the average molar mass of air (~29 g/mol), so it sinks and is collected by upward delivery (the gas enters from the bottom and displaces air upward). Sodium carbonate also works, but the acid should be dilute hydrochloric acid, not sulfuric acid, because calcium sulfate is sparingly soluble and can coat the solid. However, sodium carbonate with dilute HCl also produces CO2, but the key is the collection method. Option C is correct because it uses CaCO3 and dilute HCl, and upward delivery. Option A is wrong because upward delivery is for gases lighter than air. Option B uses sodium carbonate and dilute sulfuric acid, which is not ideal because sodium sulfate is soluble, but the collection method is wrong (downward delivery is for lighter gases). Option D is wrong because CO2 is soluble in water.
Example
In lab, add marble chips (CaCO3) to dilute HCl in a flask, and collect the gas by upward delivery in an inverted bottle.
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