Call

Use the $(P_1V_1)/T_1=(P_2V_2)/T_2$.
$"STP=273.15 K and 1 atm"$

Given/Known
$P_1="0.90 atm"$
$V_1="50.0 mL"$
$T_1="298 K"$
$P_2="1 atm"$
$T_2="273.15 K"$

Unknown
$V_2$

Solution
Rearrange the equation to isolate $V_2$ and solve.

$(P_1V_1)/T_1=(P_2V_2)/T_2$

$V_2=(P_1V_1T_2)/(T_1P_2)$

$V_2=(0.90cancel"atm"xx50.0"mL"xx273.15cancel"K")/(298cancel"K"xx1cancel"atm")="41 mL"$

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