We’re being asked to determine the molecular weight of the unknown gas.

Recall that **molecular weight** is in grams per 1 mole of a substance.

$\overline{){\mathbf{M}}{\mathbf{o}}{\mathbf{l}}{\mathbf{e}}{\mathbf{c}}{\mathbf{u}}{\mathbf{l}}{\mathbf{a}}{\mathbf{r}}{\mathbf{}}{\mathbf{W}}{\mathbf{e}}{\mathbf{i}}{\mathbf{g}}{\mathbf{h}}{\mathbf{t}}{\mathbf{}}{\mathbf{\left(}}{\mathbf{M}}{\mathbf{W}}{\mathbf{\right)}}{\mathbf{=}}\frac{\mathbf{g}}{\mathbf{m}\mathbf{o}\mathbf{l}}}$

First, we have to calculate the amount of gas in **moles** using the **ideal gas equation**.

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

P = pressure, atm

V = volume, L

n = moles, mol

R = gas constant = 0.08206 (L·atm)/(mol·K)

T = temperature, K

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

Calculate the molar mass of a gas if 2.50 g occupies 0.875 L at 685 Torr and 35°C.

(a) 0.106 g/mole

(b) 28.1 g/mole

(c) 9.12 g/mole

(d) 32.0 g/mole

(e) 80.1 g/mole

Frequently Asked Questions

What scientific concept do you need to know in order to solve this problem?

Our tutors have indicated that to solve this problem you will need to apply the Ideal Gas Law Molar Mass concept. If you need more Ideal Gas Law Molar Mass practice, you can also practice Ideal Gas Law Molar Mass practice problems.

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