First of all finding the pressure, again we will be assuming an ideal pressure of 1 atm, this will be our total pressure, now we have to find the partial pressure for H2. The partial pressure for H2O is related to its temperature, when it's 40°C we will have a pressure of 55.37 torr, which is 0.0728 atm
Ptotal = Ph2 + Ph2o
1 atm = Ph2 + 0.0728 atm
Ph2 = 0.927 atm
Now we will add this information to our Ideal Gas law formula:
PV = nRT
0.927 * 0.081 = n * 0.082 * 313
0.075 = 25.66n
n = 0.003 moles
H2 = 2.0158g/mol
0.003 moles of H2 = 6.0*10^-3
