Balance this equation. Prove that it obeys the law of conservation of
mass by using formula masses.
HNO3 + Mg(OH)2 ---> HOH + Mg(NO3)2

Hello again

Swapping the front and back ends (so the H joins up with the (OH)2 and
the Mg joins up with the NO3, you'll see you need a double dose of
HNO3 and it will balance:
2 HNO3 + Mg(OH)2 ---> 2HOH + Mg(NO3)2

Conservation of mass just means that sum of the the molecular weights
on the left is the same as the sum of the molecular weights on the
right

2 HNO3 = 2 * (1 + 14 + 3 * 16) = 126
Mg(OH)2 = 24 + 2 * (16 + 1) = 58
 Total = 184

2 HOH = 2 * (1+16+1) = 36
Mg(NO3)= 24 + 2 * (14 + 3*16)  = 148
  Total = 184

http://www.dartmouth.edu/~chemlab/info/resources/p_table/Periodic.html

Search terms used
"periodic table"

Thanks for letting us help!
Richard-ga