chemistry partial pressure

Question

In an experiment, 28.6mL of hydrogen gas were collected over water at a total pressure of 762 torr at a temperature of 15 C. If the vapor pressure of water at 15 C is 13 torr, what mass of hydrogen was obtained?

Answer 1

The pressure due to hydrogen gas is the total pressure less the vapor pressure of water. So, to find mass of H₂, we will first find the pressure of H2 and then using the ideal gas law, we'll find the moles of H₂ and then the mass of H₂.

PV = nRT

P = pressure of H2 = 762 torr - 13 torr = 749 torr x 1 atm/760 torr = 0.9855 atm (converted to atm)

V = volume in L = 28.6 ml x 1 L/1000 ml = 0.0286 L

n = moles of H₂ = ?

R = gas constant = 0.0821 Latm/Kmol

T = temp in Kelvin = 15 +273 = 288K

Solving for n, we have...

n = PV/RT = (0.9855)(0.0286)/(0.0821)(288)

n = 0.001192 moles H₂

Mass of H₂ = 0.001192 moles H₂ x 2 g/mol = 0.00238 g = 2.38 mg