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Pressure Over Water Formula:
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The pressure over water calculation determines the partial pressure of a dry gas collected over water by subtracting the water vapor pressure from the total pressure. This is important in gas collection experiments where water vapor contributes to the total pressure measurement.
The calculator uses the simple formula:
Where:
Explanation: When a gas is collected over water, the total pressure is the sum of the partial pressure of the dry gas and the vapor pressure of water. Subtracting the water vapor pressure gives the true pressure of the dry gas.
Details: Accurate calculation of dry gas pressure is crucial in chemistry experiments, particularly in gas collection methods, stoichiometry calculations, and determining gas volumes under standard conditions.
Tips: Enter the total pressure and water vapor pressure in Pascals (Pa). Both values must be positive, and the total pressure must be greater than or equal to the water vapor pressure.
Q1: Why do we need to subtract water vapor pressure?
A: When gases are collected over water, water vapor contributes to the total pressure. To find the true pressure of the dry gas, we must subtract the water vapor pressure.
Q2: How do I find the water vapor pressure?
A: Water vapor pressure depends on temperature. You can find it in standard reference tables or calculate it using appropriate formulas based on the temperature of the water.
Q3: Can I use different pressure units?
A: Yes, but all values must be in the same units. This calculator uses Pascals, but you can convert from mmHg, atm, or other units before calculation.
Q4: What if the total pressure is less than water vapor pressure?
A: This is not physically possible in normal conditions. The calculator will show an error if this occurs.
Q5: Is this calculation temperature-dependent?
A: Yes, water vapor pressure varies with temperature, so the temperature at which the gas was collected is important for accurate results.