At a pressure of 1 atmosphere and a temperature of 25 °C, 500 cm3 of the gas HX has a mass of 0.400 g. Calculate the value of the relative atomic mass of X and hence identify element X. (You should assume that 1 mole of gas occupies 24.5 dm3, and 1 dm3 = 1000 cm3. Assume the relative atomic mass of hydrogen to be 1.0.)