A piece of solid magnesium is reacted with hydrochloric acid to form hydrogen gas:

Mg (s) + 2HCl (aq) ---> MgCl2 (aq) + H2 (g)

What volume of hydrogen gas (in mL) is collected over water at 25 degrees Celsius by reaction of 0.450 g of Mg (AW= 24.3 g/mol) with 50.0 mL of 1.0 M HCl? The barometer records an atmospheric pressure of 758 torr and the vapor pressure of water at this temperature is 23.35 torr.

mols Mg = grams/molar mass = ?

Using the coefficients in the balanced equation, convert mols Mg to mols H2. That is a 1:1 so mols Mg = mols H2.
Then use PV = nRT to solve for volume in L and convert to mL.
For n use mols from above.
R and T are the usual.
For P use
Ptotal = pH2 + pH2O
758 = pH2 + 23.35
Solve for pH2 in torr and convert that to atmospheres.