Question

In a certain experiment to determine the amount of zinc in a penny 95.1 mL of hydrogen gas was collected over water at 748 torr and 25°C use this information to calculate the mass in grams of zinc in the penny

Answer 1

Volume of H2 gas collected V = 95.1 mL x 1L/1000 mL

= 0.0951 L

Pressure of dry gas = Barometric pressure - Vapor pressure of water

= 748 - 23.8 = 724.2 torr

= 724.2 torr x 1 atm/ 760 Torr = 0.9528 atm

Temperature T = 25 + 273 = 298 K

Moles of H2 gas collected n = PV/RT

=( 0.9528 atm x 0.0951 L) / (0.0821 L-atm/mol-K x 298 K)

= 0.0037039 mol

Balanced equation

Zn + 2HCl = ZnCl2 + H2

Moles of Zn reacted = moles of H2 collected

= 0.0037039 mol

Mass of zinc = moles x molecular weight

= 0.0037039 mol x 65.38 g/mol

= 0.242 g