Question

a 5.00 l flask contains 7.94 g of a gas at stp. what is the molar mass of the gas?
a 179 g/mol
b 14.1 g/mol
c 1.59 g/mol
d 35.6 g/mol

Explanation:

Step1: Find moles of gas at STP

At STP, 1 mole of any gas occupies 22.4 L. So, moles ($n$) = volume / 22.4 L/mol.
Given volume = 5.00 L, so $n = \frac{5.00\ \text{L}}{22.4\ \text{L/mol}} \approx 0.2232\ \text{mol}$.

Step2: Calculate molar mass

Molar mass ($M$) = mass ($m$) / moles ($n$).
Given mass $m = 7.94\ \text{g}$, so $M = \frac{7.94\ \text{g}}{0.2232\ \text{mol}} \approx 35.6\ \text{g/mol}$.

Answer:

d. 35.6 g/mol