Real and Ideal Gases - Physical Chemistry

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Question

Given the ideal gas law:

\ P = \rho $\frac{R}{M}$T

Where is density, is pressure, is the gas constant, is molar mass, and is temperature.

Based on the Ideal gas law, which of the following are true?

I. Pressure and volume are inversely proportional

II. Pressure and density are inversely proportional

III. Pressure and temperature are directly proportional

IV. Density and temperature are inversely proportional

V. R and M are inversely proportional

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Answer

Condition I is true. Pressure and volume are inversely proportional. The ideal gas law with volume can be rederived to show this:

We introduce an expression for moles:

 n = {$\frac{m}{M}$}

Where is moles, is mass, and is molar mass in

Rearranging (1) to solve for and plugging into the following:

\ P = \rho $\frac{R}{M}$T

We get:

(2)

Finally, we use the following for density, where is volume, and plug into (2)

The masses cancel out and we have the ideal gas law expressed with volume and moles.

Condition II is false. It is clear from the equation that pressure and density are directly proportional.

Condition III is true because it is clear from the equation that temperature and pressure are directly proportional.

Condition IV is true because density and temperature are on the same side of the equation in the numerator, so the must be inversely proportional.

Condition V is false because the ideal gas constant and molar mass can be rearranged to be on opposite sides of the equation and in the numerator.

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