Hypothesis: in the reaction Zn + 2 HCl → ZnCl₂ + H₂, the total mass after the reaction equals the total mass before it, provided no hydrogen is allowed to escape.
The design problem. Hydrogen is the lightest of all gases and it comes off fast. So the whole experiment turns on one thing: the system must be sealed, and it must be sealed before the zinc touches the acid.
Apparatus: a 250 mL conical flask, a small test tube that fits inside it, a balloon and thread (or a rubber stopper carrying a small balloon), granulated zinc, dilute hydrochloric acid, and a digital balance reading to 0.01 g.
Method
- Put about 25 mL of dilute HCl into the conical flask.
- Put about 1.3 g of granulated zinc into the small test tube and lower the test tube upright into the flask, so that the zinc cannot yet fall into the acid.
- Stretch the balloon over the mouth of the flask and tie it on tightly with thread.
- Place the whole sealed assembly on the balance and record the initial reading, m₁.
- Without lifting the flask off the pan, tilt it so that the test tube topples and the zinc falls into the acid. Bubbles appear at once and the balloon starts to inflate.
- Wait until the fizzing stops completely, then record the final reading, m₂.
- Repeat the whole run three times.
Why 1.3 g of zinc? Because you must produce enough hydrogen for the balance to notice.
Zn + 2 HCl → ZnCl₂ + H₂
65 u of Zn gives 2 u of H₂
Mass of H₂ from 1.3 g of Zn = 1.3 g × 2 ÷ 65 = 0.04 g
That is four times the 0.01 g resolution of the balance, so an open-flask control run would show a clear, readable loss. Check the mass balance of the equation itself:
Products: 136 u (ZnCl₂) + 2 u (H₂) = 138 u ✓
Expected result
| Run | Initial reading m₁ | Final reading m₂ | m₂ − m₁ |
|---|---|---|---|
| 1 (sealed) | 176.42 g | 176.42 g | 0.00 g |
| 2 (sealed) | 178.05 g | 178.04 g | −0.01 g |
| 3 (open control) | 175.90 g | 175.86 g | −0.04 g |
Conclusion: in the sealed runs m₂ = m₁ to within ±0.01 g — the uncertainty of the last digit — so mass is conserved. The open control loses 0.04 g, exactly the calculated mass of hydrogen, which shows that the “loss” in an open vessel is a loss of gas from the pan and not a loss of matter from the universe.