Using the **ideal gas equation**.

$\overline{){\mathbf{P}}{\mathbf{V}}{\mathbf{=}}{\mathbf{n}}{\mathbf{R}}{\mathbf{T}}}$

**Isolate n (number of moles of gas): **

The mass of an evacuated 255-mL flask is 143.187 g. The mass of the flask filled with 268 torr of an unknown gas at 25 ^{o}C is 143.290 g .

Calculate the molar mass of the unknown gas.

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