dalton's law of partial pressure

Question

A 1.00  flask is filled with 1.45  of argon at 25 . A sample of ethane vapor is added to the same flask until the total pressure is 1.000  .

What is the partial pressure of ethane, P ethane in the flask?

Express your answer to three significant figures and include the appropriate units.

Answer 1

Partial pressure of a gas is the pressure that it would exert if it were there alone. You didn't include units for the argon gas, so I'll assume it is grams of argon.

1.45 g Ar x 1 mol Ar/39.9 g = 0.0363 moles Ar

Pressure of Ar: PV = nRT and solving for P we have P = nRT/V = (0.0363)(0.0821)(298K)/1.00 = 0.889 atm

Since the initial pressure is 0.889 atm and the final pressure is 1.00 atm, the difference must be due to the added ethane vapor. Thus, 1.000 - 0.889 = 0.112 atm of ethane