# Question 5f515

Dec 12, 2015

$\text{170 g/mol}$

#### Explanation:

What you need to do here is use the molar volume of a gas at STP to determine how many moles of gas you have in that $\text{80-mL}$ sample, then use the given mass to determine the gas' molar mass.

STP conditions are defined for a pressure of $\text{100 kPa}$ and a temperature of ${0}^{\circ} \text{C}$. Under these conditions, one mole of any ideal gas occupies exactly $\text{22.7 L}$ - this is known as the molar volume of a gas at STP.

Convert the volume given to you from milliliters to liters first

80 color(red)(cancel(color(black)("mL"))) * "1 L"/(1000color(red)(cancel(color(black)("mL")))) = "0.080 L"

So, if one mole occupies $\text{22.7 L}$ at STP, it follows that you will have

0.080 color(red)(cancel(color(black)("L"))) * "1 mole"/(22.7 color(red)(cancel(color(black)("L")))) = "0.003524 moles"#

of gas in your $\text{0.6-g}$ sample. As you know, molar mass is defined as the mass of one mole of a substance. This means that you gas will have a molar mass of

${M}_{M} = \text{0.6 g"/"0.003524 moles" = "170.26 g/mol}$

You should round this off to one sig fig, the number of sig figs you have for your values, but I'll leave the answer rounded to two sig figs

${M}_{M} = \textcolor{g r e e n}{\text{170 g/mol}}$