Using the ideal gas equation,
PV = nRT.
where \(n = \frac{mass}{\text{molar mass}}\).
For Hydrogen, \(\frac{m_H}{M_H} = \frac{PV}{RT}\).
For element X, \(\frac{m_X}{M_X} = \frac{PV}{RT}\)
Under the same conditions implies \(\frac{PV}{RT}\) is the same for both hydrogen and X.
\(\frac{m_H}{M_H} = \frac{m_X}{M_X}\)
Molar mass of X = \(\frac{\text{molar mass of hydrogen} \times \text{mass of X}}{\text{mass of hydrogen}}\)
= \(\frac{1 \times 9.6}{0.3}\)
= 32 g/mol