Question

1.92 g M+ ion reacts with 0.158 mol X- ion to produce compound, MX2, which is 86.8% X by mass. What are the identities of M+ and X- ?

Answer 1

M2+ + 2X- ---> MX2

now that you know the equation all you gotta do is transfer the moles of X to M by doing this:

0.158mol / 2 * 1 = 0.079 mol of M2+

now that you know moles you can figure out the molar mass by doing this

M = m / n
M = 1.92g / 0.079 mol
M = 24.3 g/mol

By looking at the periodic table you can see that this belongs to Magnesium. So your M ion is Magnesium

Now to find out X you use the mass ratio given to you. And since you know the mass ratio of X you can figure out the mass ratio of M by subtracting 86.8 from 100

100 - 86.8 = 13.2%

Then from that you can do this

If 13.2% of the total mass is 1.92g then the rest of the mass is:

1.92g / 13.2 *100 = 14.5g

Mass of X is
14.5g - 1.92g =12.58g

Molar mass of x is
12.58g / 0.158 mol = 79.62g/mol

Looking at the periodic table you can see that is bromine:

So your compound is MgBr2