vapor pressure?

Question

A sample of nitrogen gas was collected over water at 20.°C and a total pressure of 1.40 atm. A total volume of 2.35 102 mL was collected. What mass of nitrogen was collected? (At 20.°C the vapor pressure of water is 17.5 torr.)

Answer 1

You want to find a number of moles N2 so that you can multiply that by the molar mass of N2 to find the mass in grams..

The equation that you always use to find number of moles of gas is your old friend,

PV = nRT

Here P = pressure due to N2 = total pressure - vapour pressure of water, and you are told V and T.

Remember that your units need to match. For example, if you use R in L atm K-1 mol-1, P must be in atm and V in L. Also notice that you are given the vapour pressure of water in torr, so that you need to convert it to atm for consistency.

Now all you need to do is the arithmetic.