Problem: A 500 mL gas sample is collected over water at a pressure of 740 mmHg and 25°C. What is the volume of the dry gas at STP? (STP= 1 atm and 0°C.)Vapor Pressure of H2O at 25°C: 24 mmHg 

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We’re being asked to determine the volume of the gas sample collected over water at STP (standard temperature and pressure).

At STP:         T = 0°C

                      P = 1 atm


We’re going to calculate the volume of the gas using the ideal gas equation.

PV=nRT

P = pressure, atm
V = volume, L
n = moles, mol
R = gas constant = 0.08206 (L·atm)/(mol·K)
T = temperature, K

The amount of gas (n) will not change and R is a constant:

PV=nRTPVT=TTPVT=1


Relating the two conditions:

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Problem Details

A 500 mL gas sample is collected over water at a pressure of 740 mmHg and 25°C. What is the volume of the dry gas at STP? (STP= 1 atm and 0°C.)

Vapor Pressure of H2O at 25°C: 24 mmHg

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