Question

A volume of 227 mL of hydrogen is collected over water; the water level in the collecting vessel is the same as the outside level. Atmospheric pressure is 756.0 Torr and the temperature is 25 °C. The vapor pressure of water at various temperatures can be found in this table. Calculate the molar mass of the metal.

Answer 1

Metal reacts with H2SO4

M (s) + H2SO4 (aq) = MSO4 + H2 (g)

The vapor pressure of water at 25 C = 23.8 torr
Atmospheric pressure = pressure by H2 gas + vapor pressure of water
756.0 = x + 23.8

x = 732.2 mmHg
From the ideal gas equation
PV = nRT

(732.2 torr x 1atm/ 760torr ) (0.227 L) = (n) (0.08206 L atm / mol K) (298 K)

n = 0.00894 moles of H2

There is a 1 to 1 molar ratio between M and H2

Moles of metal M reacted = 0.00894 moles

Molar mass of metal = mass/moles

We assume the mass of metal = 0.217 g (as it is not given)

= 0.217 g / 0.00894 moles

= 24.3 g/mol

The metal M is magnesium.