2.3: PASS Ideal Gases - partial pressure, gas over water calculation (2.E.36)
- Page ID
- 465476
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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.
- Answer
-
741 torr
- Strategy Map
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Step Hint 1. Identify what gas the question is asking you to find the pressure for. You have a gas mixture. See LibreText 2.4.3 The pressure of a mixture of gases. 2. Identify the total pressure and the vapour pressure. Recall vapour pressure is the partial pressure of water (see LibreText section 2.4.3.1)
3. Use the sum of the partial pressures equation. - Solution
-
\(\begin{gathered}
P_{\text {tot }}=P_{\text {gas }}+P_{\mathrm{H}_2 \mathrm{O}} \\
756 \mathrm{torr}=P_{\text {gas }}+15.5 \mathrm{torr} \\
756 \mathrm{torr}-15.5 \mathrm{torr}=p_{\text {gas }} \\
P_{\text {gas }}=\mathbf{7 4 0 .5} \text { torr } \\
\quad \text { or } \\
P_{\text {gas }}=\mathbf{7 4 1} \text { torr }
\end{gathered}\) - Guided Solution
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Download Guided Solution as a pdf
Guided Solution Hint This 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. See LibreText 2.4.3 The pressure of a mixture of gases. What does it mean by collected over water?
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.
\(P_{\text {tot }}=P_{g a s}+P_{\mathrm{H}_2 \mathrm{O}}\)
Complete Solution:
Recall that the sum of your partial pressures is equal to your total pressure.
\(P_{\text {tot }}=P_{g a s}+P_{\mathrm{H}_2 \mathrm{O}}\)
Plug in the values provided in the question.
\(756 \text { torr }-15.5 t \mathrm{torr}=p_{\text {gas }}\)
Isolate and solve for your desired variable.
\(\begin{gathered}
756 \mathrm{torr}-15.5 \mathrm{torr}=p_{\text {gas }} \\
\boldsymbol{P}_{\text {gas }}=\mathbf{7 4 0 . 5} \mathrm{torr} \\
\quad \text { or } \\
\boldsymbol{P}_{\text {gas }}=\mathbf{7 4 1 t o r r}
\end{gathered}\)
Answer 741 torr
The answer should have no decimal places since the one of the values in the question does not have any digits after the decimal.
Check your work!
Your partial pressure of carbon dioxide should be less than your total pressure.
Why does this answer make chemical sense?
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 its 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.
If your CO2 pressure is higher than the total pressure then you have likely added instead of subtracting the water vapour pressure.
(question source from page titled 9.11: Exercises, shared under a CC BY 4.0 license platform https://chem.libretexts.org/Bookshelves/General_Chemistry/Chemistry_2e_(OpenSTAX)/09%3A_Gases/9.11%3A_Exercises, shared under a CC BY 4.0 license, authored, remixed, and/or curated by OpenStax, original source https://openstax.org/books/chemistry-2e/pages/9-exercises), Access for free at https://openstax.org/books/chemistry/pages/1-introduction)