Gaseous hydrazine reacts with a gas X to form the gases nitrogen and HX
by way of the following reaction:
N2H4(g) + 2X2 = N2(g) + 4HX(g) (1)
(I)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.)
(ii) The enthalpy change for Reaction 1 is −1180 kJ mol−1
Sketch a fully .
labelled diagram showing how the internal energy changes for this
reaction. On your diagram indicate the energy barrier for the reaction.

I'll take a stab at part (I):
If 1 mol of gas occupies 24.5 L, then 0.5 L of gas must be 1/49 of a mol. Therefore, a mol of the gas HX would have a mass of 49 * 0.4 = 19.6 g. We know that each molecule of the gas contains a hydrogen atom (with an atomic weight of about 1.0), which means that 1 mol of atoms of element X must have a mass of 19.6 - 1.0 = 18.6 g. The element with an atomic weight closest to that is Fluorine.

Hum... I suggest you take a look at this thread:

https://www.askmehelpdesk.com/chemistry/help-me-chemistry-question-548405.html