The volume of hydrogen gas collected over water was 87.5 mL at 20 degrees C and a barometer reading of 763 mm Hg. How do you calculate the STP molar volume for hydrogen?
Well, first off you need to quote the saturated vapour pressure,
It is convenient in the laboratory to collect a volume of gas in a graduated cylinder that had previously been filled with water and inverted. The water inside the cylinder may be displaced by an added gas, and the pressure inside the cylinder equilibrated with the laboratory pressure. It's a bit awkward, but you can do it.
Now, the gas inside the cylinder is the gas from the experiment, and the
And then you need some more data. You need a mass or molar quantity of the gas; typically this is done by taking the mass of magnesium metal that you used to generate the dihydrogen. Given either one, you can calculate a molar volume. So your question is incomplete.