Answer
$1.26\text{ mol}$
Work Step by Step
$\frac{n_1*T_1}{P_1}=\frac{n_2*T_2}{P_2}$
$\frac{1.50*298}{400.}=\frac{n_2*323}{800.}$
$n_2=2.76\text{ mol}$
$2.76\text{ mol}-1.50\text{ mol}=1.26\text{ mol}$
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