Question

What is the partial pressure of the dry gas, when that gas is produced in a chemical reaction over water at 20.0 °C? The atmospheric pressure is 0.987 atm., and the pressure of water vapor at 20.0 °C is 17.5 mmHg.

Answer 1

To find the partial pressure of a dry gas collected over water, subtract the vapor pressure of water from the total atmospheric pressure. At 20.0 °C and an atmospheric pressure of 0.987 atm with a water vapor pressure of 17.5 mmHg, the dry gas pressure is 0.964 atm.

Step-by-step explanation:

The question asks about the partial pressure of a dry gas collected over water. To find this value, one must consider the total atmospheric pressure and subtract the vapor pressure of water at the given temperature.

Using the data provided:

• The atmospheric pressure is 0.987 atm
• The vapor pressure of water at 20.0 °C is 17.5 mmHg, which needs to be converted to atm (1 atm = 760 mmHg). Hence, 17.5 mmHg is approximately 0.023 atm (17.5 mmHg / 760 mmHg/atm)
• Applying Dalton's Law of partial pressures, the partial pressure of the dry gas alone (dry gas pressure) is the total atmospheric pressure minus the vapor pressure of the water: 0.987 atm - 0.023 atm = 0.964 atm.

Therefore, the partial pressure of the dry gas is 0.964 atm.