a student collected a sample of hydrogen gas by displacement of water. Volume of

Question

a student collected a sample of hydrogen gas by displacement of water. Volume of sample = 90.0 mL
Temperature = 25 C atmospheric pressure = 745 mm equilibrium vapor presssure of water at 25C = 23.8 mm Hg Calculate number of moles of hydrogen gas collected

Explanation / Answer

partial pressure of the hydrogen= the atmospheric pressure - the vapor pressure of water => 745 mm Hg - 23.8 mm Hg = 721.2 mm Hg Then convert this to atm to use in the equation PV = nRT =>721.2 mm Hg x 1 atm/760 mm Hg = 0.9489 atm the volumes of hydrogen and water vapor are proportional to their partial pressures volume of the hydrogen= multiplying the total volume (90 mL) by the partial pressure of hyrdogen divided by the atmospheric pressure =>90 mL * (721.2 mm Hg/745 mm Hg) = 87.12 mL = 87.12 x 10^-3 L (converted to L for use in PV=nRT) Then use these values to find n in PV=nRT 0.9489atm * 87.12x10^-3L = n * 8.31 * 308K 0.082677 = 2559.48 * n n = 3.23x10^-5 mol H2

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