The answer is 186.56 g.
The molar mass (Mr) of Mg(OH)₂ is the sum of atomic masses (Ar) of the elements.
Ar(Mg) = 24.3 g/mol
Ar(H) = 1
Ar(O) = 16
Mr(Mg(OH)₂) = Ar(Mg) + 2Ar(O) + 2Ar(H) = 24.3 + 2*1 + 2*16 = 24.3 + 2 + 32 = 58.3 g/mol
Now, make a proportion. If 58.3 g are in 1 mol, how much will be in 3.2 mol:
58.3 g : 1 mol = x : 3.2 mol
x = 58.3 g · 3.2 mol ÷ 1 mol = 186.56 g
