Question

Question #5f515

Answer 1

`"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 `"80-mL"` sample, then use the given mass to determine the gas' molar mass.

STP conditions are defined for a pressure of `"100 kPa"` and a temperature of `0^@"C"`. Under these conditions, one mole of any ideal gas occupies exactly `"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 `"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 `"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 = "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 = color(green)("170 g/mol")`