# How would you find the molecular weight of an unknown gas?

I would determine the mass of a fixed volume of the gas at a known temperature and pressure and then use the Ideal Gas Law to calculate the molar mass.

AN EXAMPLE

The mass of the flask filled with an unknown gas at 25.0 °C and 265 Torr is 143.289 g, while the mass of an evacuated 255 mL flask is 143.187 g.

Resolution

The ideal gas law is

We can change things around to get

There are 28.1 g/mol of molar mass.

∴ 28.1 u is the molecular mass.

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To find the molecular weight of an unknown gas, you can use the ideal gas law equation: (PV = nRT), where (P) is pressure, (V) is volume, (n) is the number of moles, (R) is the ideal gas constant, and (T) is temperature. Rearrange the equation to solve for (n), the number of moles. Then, use the equation (n = \frac{m}{M}), where (m) is the mass of the gas and (M) is the molar mass. Solving for (M) gives the molecular weight of the unknown gas.

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When evaluating a one-sided limit, you need to be careful when a quantity is approaching zero since its sign is different depending on which way it is approaching zero from. Let us look at some examples.

When evaluating a one-sided limit, you need to be careful when a quantity is approaching zero since its sign is different depending on which way it is approaching zero from. Let us look at some examples.

When evaluating a one-sided limit, you need to be careful when a quantity is approaching zero since its sign is different depending on which way it is approaching zero from. Let us look at some examples.

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