When an atom becomes an ion does its mass number change?

Its mass number does not change because the atom is losing or gaining electrons, so the change is almost not measurable because the electrons weigh so little. 

In other words: its mass number does not change

0.0738 moles

Explanation:

1) Equilibrium chemical equation:

H₂(g) + X₂(g) ⇄ 2HX(g)

2) Equilibrium constant equation:

3) Concentrations:

Molarity = number of moles of sulute / volume of solution in liters

[H₂] = 0.200 mol / 4.00 liter = 0.0500 M

[HX] = 0.600 mol / 4.00 liter = 0.150 M

4) Replace the known concentrations into the Kc equation:

5) Solve for [X₂]

5) Convert the concentration to number of moles

number of moles = M×V = 0.0184M×4.00liter = 0.0738 moles