Gas Collected Over Water

Question

Consider the reaction of a piece of zinc reacting with hydrochloric acid. The gas is collected above water based on the data below.

a. Write the reaction occurring.

   Zn(s) + 2HCl(aq) ----> ZnCl₂(aq) + H₂(g)

b. Determine the pressure of the gas formed.

c. Determine the volume (mL) of gas collected

Mass of Zn 0.032 g

Total Pressure 30.00 in Hg

Water Temperature 20.0°C

Answer 1

a). Zn(s) + 2HCl(aq) ==> ZnCl₂(aq) + H₂(g) ... balanced equation

b). P = total pressure - vapor pressure of water at 20.0ºC = 30.00 mm Hg - 17. mm Hg = 12.5 mm Hg H₂

c). PV = nRT and solve for V = nRT/P

n = moles of H₂ = 0.032 g Zn x 1 mol Zn/65.39 g x 1 mol H₂/mol Zn = 0.0004894 mol H₂ (assuming HCl in xs)

R = gas constant = 62.36 L-mm Hg/Kmol

T = temperature in K = 20 + 273 = 293K

P = pressure of H₂ gas = 12.5 mm Hg

V = (0.0004894)(62.36)(293) / 12.5

V = 0.715 L = 715 ml of H₂ gas