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): **

An unknown gas contained in a 1.85 L flask is weighed and found to have a mass of 15.87 g at a pressure of 2.45 atm and a temperature of 18.6°C. What is the molecular weight of the gas?

a. 5.35 g/mol

b. 83.8 g/mol

c. 1.14 g/mol

d. 155 g/mol

e. 77.1 g/mol

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