How do you complete the acid-base(neutralization) reaction?

Question

HF+Mg(OH)2--->________ + H2O

H3PO4+NH4OH--->_________+H2O

and also what is the pH of solution with a [H+]=4.5*10^-9??

Answer 1

You balance an acid-base reaction the same way as any other equation.

pH = -log[H+]

Answer 2

2HF + Mg(OH)2 → MgF2 + 2H2O

H3PO4 + 3NH4OH → (NH4)3PO4 +3H2O

The trick is to remember that a base plus an acid gives a salt and water.

pH = -log [H+]

pH = -log 4.5*10^-9

pH = 8.35