\(\text { moles }=\frac{ x }{280}\)
heat released by \(\frac{ x }{280}\) mole \(=2.5 \times 0.45\,kJ\)
heat released by \(1\,mole\) \(=\frac{2.5 \times 0.45 \times 280}{ x } kJ\)
\(\Delta H =\Delta U +\Delta ngRT\)
\(\Delta H \simeq \Delta U\)
\(9=\frac{2.5 \times 280 \times 0.45}{ x }\)
\(x =35\,g\)