A 1.00 L flask is filled with Ar by water displacement at 20.0°C. the atmospheric pressure is 759.6 torr. after correcting for water vapor pressure in the flask, determine how many moles of Ar are present

Answer 1

Use PV = nRT and solve for n.
Look up the vapor pressure of H2O at 20 C. It is approximately 20 torr BUT that's just a guess (although a very close guess).
Ptotal = pH2O + pAr
You know ptotal and pH2O, solve for pAr and use that for P in PV = nRT. Substitute the other numbers and solve for n. Don't forget T must be in kelvin.