Ideal Gases: Partial Pressure, Gas Over Water Calculation
Question
A sample of carbon monoxide was collected over water at a total pressure of 756 torr and a temperature of 18°C. What is the pressure of the carbon monoxide? The vapour pressure of H2O at 18°C is 15.5 torr.
Show/Hide Answer
741 torr
Refer to Section 2.4: Stoichiometry of Gaseous Substances, Mixtures, and Reactions (1).
Strategy Map
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Check out the strategy map.
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| Strategy Map Steps |
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| 1. Identify what gas the question is asking you to find the pressure for. |
| 2. Identify the total pressure and the vapour pressure. |
| 3. Use the sum of the partial pressures equation. |
Solution
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Check out this solution.
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[latex]\begin{aligned} \mathrm{P}_{\text {tot }}&=\mathrm{P}_{\text{gas }}+\mathrm{P}_{\mathrm{H}_2\mathrm{O}}\\ 756\text{ torr }&=\mathrm{P}_{\text{gas }}+15.5\text{ torr }\\ 756\text{ torr }-15.5\text { torr }&=\mathrm{P}_{\text{gas }}\\ \mathbf{P}_{\text{gas }}&=\mathbf{740.5}\mathbf{~torr}\\ &\text{ or }\\ \mathbf{P}_{\text{gas }}&=\mathbf{741}\mathbf{~torr} \end{aligned}[/latex]
Answer: 741 torr
Guided Solution
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The guided solution below will give you the reasoning for each step to get your answer, with reminders and hints.
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| Guided Solution Ideas |
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This question is a theory type problem where you must use your knowledge of partial pressures to find the pressure of carbon monoxide gas when it is collected over water.
Show/Hide Resource
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What does it mean by collected over water?
Show/Hide Don’t Forget!Recall that when a gas is collected over water, you have a mixture of the collected gas and water vapour. The vapour pressure (the partial pressure of water) must be subtracted from the total pressure. |
Recall how to calculate the SUM of all partial pressures.
Show/Hide Don’t Forget![latex]\begin{gathered} \mathrm{P}_{\mathrm{tot}}=\mathrm{P}_{\mathrm{gas}}+\mathrm{P}_{\mathrm{H}_2\mathrm{O}} \end{gathered}[/latex] |
| Complete Solution |
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| Recall that the sum of your partial pressures is equal to your total pressure:
[latex]\begin{gathered} \mathrm{P}_{\mathrm{tot}}=\mathrm{P}_{\mathrm{gas}}+\mathrm{P}_{\mathrm{H}_2\mathrm{O}} \end{gathered}[/latex] Plug in the values provided in the question: [latex]\begin{gathered} 756\mathrm{~torr}=\mathrm{P}_{\mathrm{gas}}+15.5\mathrm{~torr} \end{gathered}[/latex] Isolate and solve for your desired variable: [latex]\begin{gathered} 756\mathrm{~torr}-15.5\mathrm{~torr}=P_{\text {gas }}\\ \mathbf{P}_{\text {gas }}=\mathbf{740.5} \mathrm{~torr}\\ \mathbf{P}_{\text {gas }}=\mathbf{741}\mathbf{~torr} \end{gathered}[/latex] Answer: 741 torr |
Check Your Work
Summary of what we would expect based on the related chemistry theory.
Show/Hide Watch Out!
If your CO2 pressure is higher than the total pressure, then you have likely added instead of subtracting the water vapour pressure.
Does your answer make chemical sense?
Show/Hide Answer
When you are collecting a gas over water, that water is constantly changing between phases from liquid to solid. When you collected your desired gas, you are collecting both the desired gas as well as some water molecules in their gaseous state.
For this reason, you must subtract the partial pressure of water (the vapour pressure) from your total calculated pressure. This will give you the pressure of only your desired gas.
PASS Attribution
- LibreTexts PASS Chemistry Book CHEM 1510/1520 (2).
- Question 2.E.36 from LibreTexts PASS Chemistry Book CHEM 1510/1520 (3) is used under a CC BY-NC-SA 4.0 license.
References
1. OpenStax. 2.4: Stoichiometry of Gaseous Substances, Mixtures, and Reactions. In TRU: Fundamentals and Principles of Chemistry (CHEM 1510 and CHEM 1520); LibreTexts, 2023. https://chem.libretexts.org/Courses/Thompson_Rivers_University/TRU%3A_Fundamentals_and_Principles_of_Chemistry_(CHEM_1510_and_CHEM_1520)/02%3A_Gases/2.04%3A_Stoichiometry_of_Gaseous_Substances_Mixtures_and_Reactions.
2. Blackstock, L.; Brewer, S.; Jensen, A. PASS Chemistry Book CHEM 1510/1520; LibreTexts, 2023. https://chem.libretexts.org/Courses/Thompson_Rivers_University/PASS_Chemistry_Book_CHEM_1510%2F%2F1520.
3. Blackstock, L.; Brewer, S.; Jensen, A. 2.3: PASS Ideal Gases – Partial Pressure, Gas Over Water Calculation (2.E.36). In PASS Chemistry Book CHEM 1500; LibreTexts, 2024. https://chem.libretexts.org/Courses/Thompson_Rivers_University/PASS_Chemistry_Book_CHEM_1510_1520/02%3A_Gases/2.03%3A_2.3_PASS_Ideal_Gases_-_partial_pressure_gas_over_water_calculation_(2.E.36).
4. OpenStax. 9.E: Gases (Exercises). In Chemistry 1e (OpenSTAX); LibreTexts, 2023. https://chem.libretexts.org/Bookshelves/General_Chemistry/Chemistry_1e_(OpenSTAX)/09%3A_Gases/9.E%3A_Gases_(Exercises).
5. Flowers, P.; Robinson, W. R.; Langley, R.; Theopold, K. Ch. 9 Exercises. In Chemistry 2e; OpenStax, 2019. https://openstax.org/books/chemistry-2e/pages/9-exercises.
