what mass of mg must you use to make 5.6x10 to the fourth power of mg(OH)2

Answer 1

It is not clear to me what are the units of your 5.6E4 is, grams, kg, moles, tons?

at any rate, find the percentage of Mg in Mg(OH)2.

percentMg= atomicmassMg/(atomicmassMg+2 atomic mass of O + 2 atomic mass H)

then, massMg needed= massMg(OH)2/precentMg