Answer:
Mg(NO₃)₂ · 4 H₂O
Step-by-step explanation:
To find the formula of the hydrate, you need to (1) determine the mass of each compound, then (2) convert the mass of each compound to moles (via their molar masses), then (3) determine the lowest coefficient for each compound, and then (4) construct the formula.
(Step 1)
Because the percents add up to 100%, we can assume that the percent of each compound is equal to the mass (g) of each compound.
67.3% Mg(NO₃)₂ = 67.3 g Mg(NO₃)₂
32.7% H₂O = 32.7 g H₂O
(Step 2)
Molar Mass (Mg(NO₃)₂): 24.305 g/mol + 2(14.007 g/mol) + 6(15.998 g/mol)
Molar Mass (Mg(NO₃)₂): 148.307 g/mol
Molar Mass (H₂O): 2(1.008 g/mol) + 15.998 g/mol
Molar Mass (H₂O): 18.014 g/mol
67.3 g Mg(NO₃)₂ 1 mole
-------------------------- x ------------------ = 0.454 mole Mg(NO₃)₂
148.307 g
32.7 g H₂O 1 mole
------------------- x ----------------- = 1.82 mole H₂O
18.014 g
(Step 3)
The moles of Mg(NO₃)₂ is the lowest among the two. To translate the technical amount of moles to the moles in the formula, you need to divide each value by the lowest mole value.
0.454 mole Mg(NO₃)₂ / 0.454 mole = 1 mole Mg(NO₃)₂
1.82 mole H₂O / 0.454 mole = 4 moles H₂O
(Step 4)
The amount of moles calculated in the last step represent the coefficient in front of the compounds. Using them, you can construct your hydrate.
Mg(NO₃)₂ · 4 H₂O