[tex]Mg(OH)_2 + 2HNO_3 ---\ \textgreater \ Mg(NO_3)_2 + 2H_2O[/tex]
That is the balanced chemical equation.
Before reaction,
milli-moles of Mg(OH)2 = 100 x 0.02 = 2
milli-moles of HNO3 = 100 x 0.05 = 5
After reaction:
Mg(OH)2 is limiting reagent. It will react completely leaving some HNO3 behind.
milli-moles of HNO3 react = 2 x coefficient of HNO3 = 2 x 2 = 4
milli-moles of HNO3 left = 5 -4 = 1
Now, total volume = 100+ 100 = 200 mL
It is asked for hydronium ion concentration, which is basically hydrogen ion conc.
1 HNO3 will dissociate to give 1 H+ ion.
so, milli-moles of H+ = 1
So, concentration of H+ or hydronium(H3O+) = 1 / 200 = 0.005
