Question

A sample of He collected over water at a temperature of 20C has a pressure of 705 torr. What is the pressure of the dry gas? The vapor pressure of water at 20 degrees * C is 17.5 torr.

Answer 1

The pressure of the dry gas : 687.5 torr

Further explanation

Dalton's law of partial pressures states that the total pressure of a mixture of gases is equal to the sum of the partial pressures of the component gases

Can be formulated:

P tot = P1 + P2 + P3 ....

The partial pressure is the pressure of each gas in a mixture

Since the gas mixture consists of He and H₂O :

P tot=P He + P H₂O

`\tt 705=P~He+17.5\\\\P~He=705-17.5=687.5~torr`