# Hydrogen is collected over water at 19C and 785 mmHg of pressure. THe total volume of the sample was 1.93L How do you calculate the volume of hydrogen at STP?

##### 1 Answer

The volume of hydrogen is 1.84 L.

#### Explanation:

This is an example of a Combined Gas Laws problem.

#color(blue)(|bar(ul((P_1V_1)/T_1 = (P_2V_2)/T_2|)#

We can rearrange this formula to get

#V_2 = V_1 × P_1/P_2 × T_2/T_1#

The vapour pressure of water at 19 °C = 16.5 mmHg.

∴ The pressure of the dry hydrogen is given by

Your data are then:

The volume is 1.84 L.