$0\mathrm{.}52g$ of a gas in a $250-mL$ flask has a pressure of $1\mathrm{.}3$atm at temperature of $31C.$

How many moles of gas are present?

$(1\mathrm{.}3$atm$)(0\mathrm{.}25L)=n(\mathrm{.}08206)(304\mathrm{.}15)$

What is the approximate molecular weight of the gas? The answer is $40$ but how?