College

Gas Laws Fact Sheet

[tex]
\[
\begin{array}{|l|l|}
\hline
\text{Ideal gas law} & PV = nRT \\
\hline
\text{Ideal gas constant} & R = 8.314 \frac{\text{L kPa}}{\text{mol K}} \text{ or } R = 0.0821 \frac{\text{L atm}}{\text{mol K}} \\
\hline
\end{array}
\]
[/tex]

Select the correct answer:

When a chemist collects hydrogen gas over water, she ends up with a mixture of hydrogen and water vapor in her collecting bottle. If the pressure in the collecting bottle is 97.1 kilopascals and the vapor pressure of the water is 3.2 kilopascals, what is the partial pressure of the hydrogen?

A. 93.9 kPa
B. 98.1 kPa
C. 100.3 kPa
D. 104.5 kPa

Answer :

Certainly! Let's solve the problem step-by-step:

When a chemist collects hydrogen gas over water, the gas collected is actually a mixture of hydrogen gas and water vapor. According to Dalton's Law of Partial Pressures, the total pressure in a container is the sum of the partial pressures of all the individual gases present.

Here's how to solve the problem:

1. Total Pressure in the Collecting Bottle:
- The total pressure inside the collecting bottle is given as 97.1 kilopascals (kPa).

2. Vapor Pressure of Water:
- The pressure exerted by water vapor in the collecting bottle is given as 3.2 kilopascals (kPa).

3. Apply Dalton’s Law of Partial Pressures:
- Dalton’s Law states that the total pressure of a mixture of gases is equal to the sum of the partial pressures of each individual gas.
- Mathematically, this can be expressed as:
[tex]\[
P_{\text{total}} = P_{\text{hydrogen}} + P_{\text{water vapor}}
\][/tex]
- We want to find the partial pressure of hydrogen ([tex]\(P_{\text{hydrogen}}\)[/tex]).

4. Calculate the Partial Pressure of Hydrogen:
- Rearrange the equation to solve for the partial pressure of hydrogen:
[tex]\[
P_{\text{hydrogen}} = P_{\text{total}} - P_{\text{water vapor}}
\][/tex]
- Substitute the known values:
[tex]\[
P_{\text{hydrogen}} = 97.1 \, \text{kPa} - 3.2 \, \text{kPa}
\][/tex]
- Perform the subtraction:
[tex]\[
P_{\text{hydrogen}} = 93.9 \, \text{kPa}
\][/tex]

Therefore, the partial pressure of the hydrogen gas is 93.9 kPa, which corresponds to option A.