Question

You collect a sample of hydrogen gss in an inverted buret by displacement of water at 23 ^oC. The buret could not be submerged deep enough in the water bath to equalize pressure. The water level in the buret was 14.65 cm above the water level in the water bath. The volume of gas in the buret was determined to be 32.60 mL.

How many moles of hydrogen are in this sample?

Answer 1

Since the density of Hg is 13.6 times that of water, 1 torr = 1mmHg = 13.6mmH2O.
14.65cm water = 146.5mm water x [1torr / 13.6mm water] = 10.77torr.

So pressure of H2 + pressure of water column = atmospheric pressure = 753 torr.
P H2 = 753torr - 10.77torr = 742.23torr.

A further correction needs to be made because the H2 gas also contains water vapor.
At 26.0C the vapor pressure of water is 21.1 torr.

So pressure H2 + vapor pressure of water = 742.6 torr.
P H2 = 742.23 - 21.1= 721.13 torr.

Calculate the appropriate value for R.
PV = nRT
R = PV/nT
R = (760torr)(22400mL) / (1mole)(273K) = 62359torr mL / moleK

Finally, moles of dry H2:
PV = nRT
n = PV / RT = (721.13torr)(32.6mL) / [(62359torr mL/moleK)*(296K) = 1.27E-3 moles.