Acetylene(C2H2), an important fuel in welding, is produced in the laboratory when calcium carbide (CaC2) reacts with water: CaC2(s) + 2H20(l) → C2H2(g) + Ca(OH)2(aq) For a sample of acetylene collected over water, total gas pressure (adjusted to barometric pressure) is 738 torr and the volume is 523 mL. At the temperature of the gas(23∘C), the vapor pressure of water is 2 1 torr. How many grams of acetylene are collected?

Answer 1

NOTE: I don't believe the vapor pressure of H2O at 23 C is 2.1 torr. It's more like 21 torr. I think you made a typo or it's wrong in the problem.
Ptotal = pgas + pH2O
738 = pgas + 21
Solve for pgas.

Then use PV = nRT and solve for n = number of moles.
Finally, n = grams/molar mass. You know molar mass and n, solve for grams acetylene.