Hydrate Notes
How to find a formula
6 Steps to Find Hydrate Formula
(goal is to solve for X)
1. Write out generic formula: salt · X H₂O
2. Find mass of water that was heated off:
– Wet salt – dry salt or the difference in mass after
heating
3. Convert g of water into moles of water using
molar mass of water (round moles to
nearest whole #)
4. Convert g of dry salt into mole of dry salt
using the molar mass of the dry salt (round
moles to nearest whole #)
6 Steps to Find Hydrate Formula
(goal is to solve for X)
If ratio of moles is 1 mole salt to # water skip to
step 6…if not go to step 5
5. Divide mole numbers by the smallest mole
number
6. X is moles of water…finish formula from step
1.
When 380 grams of hydrated copper II sulfate was heated in a
crucible, 163.2 g of the anhydrous (dry) compound is left. What
is the correct formula of the hydrate?
Step 1
CuSO₄· X H₂O
Step 2
380 g wet salt – 163.2 g dry salt=
220 g water
Step 3
220 g H₂O x (1mol H₂O /18.02g H₂O)=
12 mol H₂O
Step 4
163.2 g CuSO₄ x (1 mol CuSO₄/ 159.62 g CuSO₄) =
1 mol CuSO₄
Step 6
CuSO₄· 12 H₂O
802.5 grams of hydrated zinc sulfate was heated. The change in
mass was 352.0 grams. What is the correct formula of the
hydrate?
Step 1
ZnSO₄· X H₂O
Step 2
Change in mass was the mass of water…
802.5 grams wet salt- 352.0 grams water=
450.5 g dry salt
Step 3
352.0 g H₂O x (1mol H₂O /18.02g H₂O)=
20 mol H₂O
Step 4
450.5 g ZnSO₄ x (1 mol ZnSO₄/ 161.45 g ZnSO₄) =
3 mol ZnSO₄
Step 5
20 mol H₂O /3 = 7 H₂O
3 mol ZnSO₄ / 3= 1 ZnSO₄
Step 6
ZnSO₄· 7 H₂O