In the zinc and sulfuric acid rate experiment (Zn(s) + H₂SO₄(aq) → ZnSO₄(aq) + H₂(g)) where hydrogen is collected in an inverted 50 cm³ measuring cylinder, why is the volume of hydrogen collected less than the volume of hydrogen produced?

What the question is really asking

You are comparing two things: the total amount of $H_2$ generated by the reaction in the conical flask, and the amount of that gas that is actually captured and measured in the inverted measuring cylinder. Any loss or uncollected gas makes the measured volume smaller.

Where the “missing” hydrogen can go

• Escapes before sealing: As soon as zinc touches sulfuric acid, $H_2$ is produced. While the stopper is being inserted and the timer started, some gas can leave the flask into the air.
• Leaks around joints: If the bung or delivery tube is not perfectly airtight, some hydrogen escapes instead of traveling into the cylinder.
• Dead volume in the apparatus: Some hydrogen stays in the conical flask and in the delivery tube. Only the portion that reaches the cylinder contributes to the measured volume.

Dissolving and measurement effects

• Dissolving in solution: A small amount of $H_2$ can dissolve in the acid/water, so it is produced but not collected as a gas.
• Gas collection is not perfectly efficient: Bubbles can stick, or gas can be temporarily trapped, which also lowers the reading at a given time.

Link back to the reaction

The reaction $\text{Zn(s)} + \text{H}_2\text{SO}_4\text{(aq)} \rightarrow \text{ZnSO}_4\text{(aq)} + \text{H}_2\text{(g)}$ produces hydrogen in the flask, but the measuring cylinder only records the fraction that actually makes it through the delivery path and remains trapped for measurement.