Hydrate summary
Hydrate: •A hydrate is a compound that has a specific number of water molecules bound to its atoms.
Example: Sodium carbonate decahydrate: Na2CO3 ● 10H2O
Vocab
Hydrate: the compound with water
Anhydrate / anhydrous compound: the compound with all the water driven off
Ex.
Na2CO3●10H2O = hydrate  heat to drive off H2O  becomes anhydrous Na2CO3
Determining the formula of a Hydrate
1. Determine the mass of water and the mass of the Anhydrous compound
2. Change each of the masses to moles
3. Take moles of water divided by moles of anhydrous compound
1.
Example
Gypsum is hydrated calcium sulfate. A 4.89 g sample of this hydrate was heated. After the water
was removed, 3.87g anhydrous calcium sulfate remained. Determine the formula for this hydrate
and name the compound.
Step 1 Determine the mass of water and the mass of the Anhydrous compound
4.89 g CaSO4 ●xH2O – 3.87g CaSO4 = 1.02 g H2O
Step 2 Change each of the masses to moles
Step 3 Divide mol H2O by mol anhydrous:
Answer: CaSO4 ● 2H2O name: Calcium sulfate dihydrate
Example 2
2.
Hydrated sodium tetraborate (Na2B4O7 ● xH2O) is commonly called borax. Chemical analysis
indicates that this hydrate is 52.8% Na2B4O7 . Determine the formula and name the hydrate.
Step 1
100% - 52.8% = 47.2 % H2O
47.2 g H2O
52.8 g Na2B4O7
step 2
47.2 g H2 O = 2.6193 mol H2O
52.8 g Na2B4O7 = 0.2624 mol Na2B4O7
Step 3
2.6193 / 0.2624 = 9.99 = 10
Na2B4O7 ●10H2O name: Sodium tetraborate decahydrate
Try one:
A 5.9g sample of magnesium nitrate hydrate is heated and the final mass is 2.525 g. what is the name
and formula of the hydrate?
Ans: Mg(NO3)2● 11H2O
Magnesium nitrate dodecahydrate