Question

A student prepares a sample of hydrogen gas by electrolyzing water at 25°C. She collects 112 mL of H₂ at a total pressure of 758 mm Hg. Vapor pressure of water at 25°C = 23.76 mm Hg, calculate:

1. the partial pressure of hydrogen, (b) the number of moles of hydrogen collected

Answer 1

Solution :

• Answer (a)

total pressure of 758 mm Hg l, PH2 = 758 mm Hg

Vapor pressure of water at 25°C = 23.76 mm Hg

PH2O = 23.76 mm Hg

Total peessure = PH2 = Patm - PH2O

PH2 =758 mm Hg – 23.76 mm Hg

PH2 = 734.24 mm Hg = 0.966 atm

the partial pressure of hydrogen is 734.24 mm Hg

• Answer (b)

Volume of H2 = 112 ml = 0.112 L

Temperature = 25 c = 298 K

PV = nRT

(0.966 atm)(0.112 L) = (n)(0.0821 L·atm/mol·K)(298 K)

n = 0.00442 mol H2

the number of moles of hydrogen collected is 0.00442 mol H2