A sample of gas is collected over water at atmospheric pressure (patm = 1.00 atm). If the vapor pressure of water at the experimental temperature is 0.030 atm, what is the pressure of the dry gas (pgas)?

• A. 1.00 atm
• B. 0.97 atm
• C. 1.03 atm
• D. 0.030 atm

Answer: (B)

Verified step by step guidance

1. Understand that the total pressure of the gas collected over water is the sum of the pressure of the dry gas and the vapor pressure of water. This is because the gas collected is a mixture of the dry gas and water vapor.
2. Write the relationship between the total pressure, the pressure of the dry gas, and the vapor pressure of water as: \(P_{\text{atm}} = P_{\text{gas}} + P_{\text{H_2O}}\).
3. Identify the given values: atmospheric pressure \(P_{\text{atm}} = 1.00\) atm and vapor pressure of water \(P_{\text{H_2O}} = 0.030\) atm.
4. Rearrange the equation to solve for the pressure of the dry gas: \(P_{\text{gas}} = P_{\text{atm}} - P_{\text{H_2O}}\).
5. Substitute the known values into the equation to find \(P_{\text{gas}}\): \(P_{\text{gas}} = 1.00\ \text{atm} - 0.030\ \text{atm}\).