J.R. S. answered 03/17/21
Ph.D. University Professor with 10+ years Tutoring Experience
To find the partial pressure of O2 gas that has been collected over water, we need to subtract the vapor pressure of water from the total pressure. This will then give us the pressure due to the O2 gas.
624 mm Hg - 12 mm Hg = 612 mm Hg = Partial pressure of O2
To find the moles of O2 gas, we can use the Ideal Gas Law, PV = nRT
P = pressure = 612 mm Hg
V = volume 6.33 L
n = moles = ?
R = gas constant = 62.36 L-mm Hg/K-mol (you could use 0.0821 Latm/Kmol if you changed 612 mm to atm)
T = temperature in K = 16ºC + 273 = 289K
Solving for n (moles) we have...
n = PV/RT = (612)(6.33) / (62.36)(289)
n = 0.215 moles