When collecting gases over water, which correction must be made to determine the pressure of the collected gas?

• A. Add the vapor pressure of water to the total pressure.
• B. Subtract the vapor pressure of water from the total pressure.
• C. Multiply the total pressure by the vapor pressure of water.
• D. Ignore the vapor pressure of water, as it does not affect the measurement.

Answer: (B)

Verified step by step guidance

1. Understand that when a gas is collected over water, the total pressure measured is actually the sum of the pressure of the collected gas and the vapor pressure of water.
2. Recognize that the vapor pressure of water depends on the temperature and represents the pressure exerted by water molecules in the gas phase above the liquid.
3. Write the relationship between the total pressure, the pressure of the collected gas, and the vapor pressure of water as: \(P_{\text{total}} = P_{\text{gas}} + P_{\text{H_2O}}\).
4. To find the pressure of the collected gas alone, rearrange the equation to isolate \(P_{\text{gas}}\): \(P_{\text{gas}} = P_{\text{total}} - P_{\text{H_2O}}\).
5. Therefore, the correction involves subtracting the vapor pressure of water from the total pressure to obtain the pressure of the dry collected gas.