Suppose you want to know the formula of a hydrate of magnesium nitrate. If you heat 1.263g of the hydrate until all of the water is vaporized, the mass of anhydrous Mg(NO3)2 is 1.016g. What is the formula of the hydrate ie what is the value of x in Mg(NO3)2.xH2)? (the formula weight of anhydrous magnesium nitrate is 148.31g/mol)

Question

aaronq · Answer

How do the moles of the anhydrate (only $\sf Mg(NO_3)_2$, no water) compare to the moles of water?

The masses are:

$\sf 1.016~g$ for $\sf Mg(NO_3)_2$

1.263 g  - 1.016 g =  $\sf 0.247 ~g$ for only water

Use the formula: $\sf \large moles=\dfrac{mass}{Molar~mass}$

aaronq · Answer

btw, "Molar mass" is the same thing as "formula weight"

anonymous · Answer

am I using the molar mass of water or Mg(NO3)2 @aaronq

aaronq · Answer

Both, separately. 

Use the mass of (anhydrous) Mg(NO3)2 and divide it by it's molar mass, then use the mass of water and divide it by it's molar mass.