# The hydrogen gas is collected by displacement of water at 35.0 °C at a total pressure of 748 torr. The vapor pressure of water at 35.0 °C is 42 torr. What is the partial pressure of #H_2# if 307.8 mL of gas is collected over water from this method?

##### 1 Answer

#### Explanation:

The trick here is to **not** get distracted by the given volume of the hydrogen gas - water vapor mixture.

All you need to focus on is the **total pressure** of the gaseous mixture and the **vapor pressure** of water at that temperature.

The partial pressure of hydrogen can be calculated using **Dalton's Law of Partial Pressures**, which tells you that the total pressure of a gaseous mixture is equal to the sum of the partial pressure of each gas that's a part of that mixture.

Simply put, each gas will contribute to the total pressure of the mixture **proportionally** to the number of molecules it has in the mixture.

Mathematically, you can write this as

#color(blue)(P_"total" = sum_i P_i)#

In this case, the total pressure of the mixture will be equal to the partial pressure of water and the partial pressure of hydrogen gas

#P_"total" = P_"water" + P_(H_2)#

This of course means that the partial pressure of hydrogen gas will be

#P_(H_2) = P_"total" - P_"water"#

#P_(H_2) = "748 torr" - "42 torr" = color(green)("706 torr")#