How do you balance H3PO4 +Mg(OH)-->Mg3(PO4)2 +H20?

##2H_3PO_4+3Mg(OH)_2->Mg_3(PO4)_2+6H_2O##

Please note that since magnesium is in group 2 it has a valency of 2+ and so the formula for magnesium hydroxide is ##Mg(OH)_2## [Mistake in question]

This is a typical reaction of an acid with a base to form a salt and water. The reaction is exothermic, gives off heat, ##Delta H<0##, and may be balanced by adding balancing numbers in front, ie adding molecules, in order to ensure that the total number of atoms of each element is the same on the left and right hand sides of the equation.

Doing so we obtain :

##2H_3PO_4+3Mg(OH)_2->Mg_3(PO4)_2+6H_2O##