posted by Amy on .
A student collected H2 gas in a buret by displacing water at 22.0 °C. The buret could not be submerged deep enough in a water bath to equalize the pressure. The water level in the buret was 18.2 cm above the water level in the water bath. The volume of the gas in the buret was determined to be 38.6 mL.
a) If the atmosphere pressure was 748 torr, what is the pressure of the Hydrogen in the buret? (Density of Hg is 13.6 g/mL)
b) How many moles of Hydrogen did the student collect?
First convert that 18.2 cm to how much pressure was exerted. The pressure of a column of water 18.2 cm high is
density x gravity x height = 1.00g/cc x 9.8 m/s^2 x 0.182m = 1.78 kPa and since the height in the buret is too high, that means the pressure on the gas inside is 1.78 kPa too much. I would convert that to mm and subtract from 748. I estimate the mm Hg for 1.78 kPa to be about 13 or so mm Hg.(You can do it another way as follows: 18.2 cm = 182 mm and 182mm/13.6 = 13.4 mm Hg)
Ptotal gas = pH2O level + pH2O vapor pressure at 22C + pH2.
You know Ptotal= 748 mm
You know added pressure due to height H2O = about 13 mm (you should do that more accurately) and you can look up the vapor pressure of H2O at 22 C.
Use PV = nRT to calculate n for H2 gas.