Question

Helium is collected over water at 20.°C and 1.16 atm total pressure. What total volume of gas must be collected to obtain 0.597 g of helium? (At 20.°C the vapor pressure of water is 17.5 torr.)

Answer 1

I would do this.
pHe + pH2O = 1.16 atm. Calculate pHe alone but be careful that you use the same units.
Then use PV = nRT and solve for V for the dry He.
Using that number for n dry He, solve PV = nRT for n for H2O at 17.5 torr.

Add n H2O + n He for total mols, then
PV = nRT and use total mols for n and total pressure for P. Calculate total V.