Question

A volume of 235 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

Pressure of gas collected over water = 756 torr = 0.994 atm

Vapor pressure of water at the given temp. = 0.031 atm

So, Pressure of hydrogen gas = 0.994-0.031 = 0.963 atm

Using the equation:

PV = nRT

Putting values:

0.963*0.235 = n*0.0821*(273+25)

Solving we get:

n = 0.00925

So, moles of H₂ gas = 0.00925

Assuming the following reaction is taking place:

M + H₂SO₄ ---> MSO₄ + H₂

Also assume that 'x' grams of the metal M were taken.

So,

Moles of M = moles of H₂ produced = 0.00925

So,

MW of the metal M = Mass/moles = x/0.00925

Put in the value of 'x' and calculate MW.

Hope this helps !