General

Help with Chem Lab Molar Volume

Type of experiment: determine the molar volume of H2 at STP.
This is based on "collected over water".

You are given a piece of Mg ribbon. You steel wool it, then weigh it. You get a small-holed rubber stopper. Using a copper wire, you tie the ribbon and the stopper together.

You have a long thin test tube filled with excess HCl (so Mg is limiting) and fill the rest of the volume with filtered water. You then plug the stopper into the test tube (the reactants won't react because HCl is dense, it being on the bottom).

You also have a graduated cylinder filled with tap water prepared beforehand. You flip the whole test tube upside down and quickly put it inside the cylinder. The HCl will fall to meet the Mg ribbon, and start producing H2 (and water vapour).

The purpose is to find the molar volume of H2, and to do that you need the mol of H2 (I think) and the volume of H2.

When the meniscus of the grad cylinder and the test tube are the same, that's the total volume of the gas produced.

In summary we get:
Mg(s) + 2HCl(aq) > MgCl2(aq) + H2(g)

0.0295 g Mg is measured.
The total volume (H2 and water vapor) is 29.2 mL
The barometric pressure is 102.49 kPa (total as well, so to get H2 pressure, you subtract pressure of H2O based on temperature)
The temperature is 12.3 Celsius (or 285.3 K)

The calculated uncertainty is about 2.43%, so if I get a % deviation higher than that, it means there's random errors, else, I'm not sure what it means.

Problem:
There's 2 ways I've heard my classmates did it. Half of them did it one way, other half did it other way.
The first way was to take the moles of H2 using stoichiometry based on the mass of Mg. Then one would use ideal gas law to get the Volume, then convert it to STP. Once done that, you would divide my mol to get molar volume. So, we didn't use the volume we measured AT ALL.. This produces 0.03%, which is insignificant, which is less than the uncertainty for this lab.

Method 2 is basically what few others did. They ONLY used volume of the gas we measure, but not the mass of Mg AT ALL..I'm not sure how one would do this, but oh well. I'm not sure what % deviation I get from this.

Method 3 appeared after I asked few university friends, who said to use both (which I thought was right). To get the moles for molar volume, you use the mole stoichiometry to get the moles of H2. To get the L for molar volume, you use the volume you measured INSTEAD of the ideal gas law applied to the mol of H2. This produces a 3% deviation

Thanks for reading and helping