Question #5f515

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")#