Collecting H2 gas over Water The table below shows how the pressure of water var
ID: 814341 • Letter: C
Question
Collecting H2 gas over Water
The table below shows how the pressure of water varies by temperature:
Temp., C
Vapor pressure of water, mmHg
5.0
6.543
10.0
9.209
15.0
12.79
20.0
17.54
35.0
42.18
50.0
92.15
80.0
355.1
100.0
760.0
When Mg is added to an aqueous solution of excess HCl, the metal reacts to make MgCl2(aq) and H2 gas.
When the reaction is complete, the volume of H2 gas collected at 15 C is 101.0 mL.
The atmospheric pressure is 694 torr.
Explanation / Answer
Since the gas is collected over water, and the pressure is set equal to atmospheric pressure, then the barometric pressure = hydrogen pressure plus water pressure, so hydrogen pressure is 694 - 12.79 which is 681 mmHg. We then need to determine the number of moles of hydrogen from the ideal gas law, PV = nRT.
n = PV/RT = (694/760)(0.101L)/(0.08206)(273+15) = 0.0039 mol H2. Two moles of acid are required to react with one mole of Mg in the balanced equation, so we have 0.00195 mol Mg which we convert to grams. (0.00195mol Mg)(24.3gMg/mol) = 0.0474 g Mg