Question

Answer using Dalton’s Law of partial pressures and the combined gas law: A 40.0 mL sample of hydrogen is collected by water displacement at a temperature of 20° C. The barometer reads 751 torr. What is the volume of dry gas at STP?

Answer 1

Ptotal = Pdry H2 + pH2O @ 20 C.
Look in a table of vapor pressure for water @ 20 C. The pressure will be approximately 20 mm but that is a close estimate. Tables will be in your text or on the web.
751 = Pdry H2 + 20 mm
Pdry H2 = ?
Then use (P1V1/T1) = (P2V2/T2)
Post your work if you get stuck.