# How to Find Partial Pressures

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Partial pressure refers to the pressure a gas exerts if kept at a constant temperature in a fixed amount of space. Scientists cannot measure the partial pressure of a gas; it must be calculated using the equation derived from Dalton’s Law of Partial Pressures. The equation used to calculate partial pressure: P = (nRT)/V, where P = partial pressure; n = number of moles of the gas; R = universal gas constant; T = temperature; and V = volume.

Multiply the number of moles of the gas by the universal gas constant. R = 0.08206 (L_atm)/(mol_K).

Multiply the result of your calculation from step one by the temperature of the gas in kelvins (K).

Divide the result of your calculation from step two by the volume of the gas in liters. Since gas expands to fill any given container, the volume of the gas is equivalent to the volume of the container in which it is located.

Record the result of your final calculation. This is the partial pressure of the gas. The unit used to express partial pressure is the atmosphere (atm).

#### Tips

• Convert the temperature into kelvins from degrees Celsius or degrees Fahrenheit using the following formulas: K = degrees Celsius + 273; or K = (5/9) * (degrees Fahrenheit – 32) + 273.

#### References

• General Chemistry, Seventh Edition; Kenneth Whitten, Ph.D., Raymond Davis, Ph.D., M. Peck, Ph.D., & George Stanley, Ph.D.; 2004
• Organic Chemistry; John McMurry; 2007