Recall that standard temperature and pressure (STP) is defined as 1 atm and 0°C.

$\overline{)\mathbf{PV}\mathbf{=}\mathbf{nRT}}\phantom{\rule{0ex}{0ex}}\frac{\overline{)\mathbf{P}}\mathbf{V}}{\overline{)\mathbf{P}}}\mathbf{=}\frac{\mathbf{nRT}}{\mathbf{P}}\phantom{\rule{0ex}{0ex}}\overline{)\mathbf{V}\mathbf{=}\frac{\mathbf{nRT}}{\mathbf{P}}}$

R is constant, P and T are the same for all → compare the number of moles for each

**↑ moles (n) = ↑ volume**

**a) 10.0 g Ar**

molar mass Ar = 39.95 g/mol

$\mathbf{moles}\mathbf{}\mathbf{Ar}\mathbf{=}\mathbf{10}\mathbf{.}\mathbf{0}\mathbf{}\overline{)\mathbf{g}}\mathbf{\times}\frac{\mathbf{1}\mathbf{}\mathbf{mol}}{\mathbf{39}\mathbf{.}\mathbf{95}\mathbf{}\overline{)\mathbf{g}}}$

**moles Ar = 0.250 mol**

**b) 10.0 g Kr**

molar mass Kr = 83.798 g/mol

$\mathbf{moles}\mathbf{}\mathbf{Kr}\mathbf{=}\mathbf{10}\mathbf{.}\mathbf{0}\mathbf{}\overline{)\mathbf{g}}\mathbf{\times}\frac{\mathbf{1}\mathbf{}\mathbf{mol}}{\mathbf{83}\mathbf{.}\mathbf{798}\mathbf{}\overline{)\mathbf{g}}}$

Which gas sample has the greatest volume at STP?

a) 10.0 g Ar

b) 10.0 g Kr

c) 10.0 g Xe

d) None of the above (They all have the same volume.)

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