Formula of a Hydrate

What is a hydrate?• An ionic compound that contains water

molecules in its structure• The formula states how many mol of

water molecules per one mol of ionic compound

• Example: CuSo4 5 H20Copper sulfate pentahydrate

What is an anhydrate?• The substance that remains after the

water is removed from a hydrate• Example: CuSo4

Anhydrous Copper sulfate

Prefixes1 mono2 di3 tri4 tetra5 penta6 hexa7 hepta8 octa9 nona10 deca

Steps to find formula1. Identify what’s given:

A. Hydrate/anhydrate (go to step 2)B. Anhydrate/water (skip to step 3)

2. If given hydrate and anhydrate, subtract to get water

3. Convert mass of anhydrate and water to moles of each, respectively

4. Find the ratio of water to anhydrate (in moles)5. Write the formula; ionic compound followed

by ratio of water6. Name the compound

Example 1

Step 2: 4.72g hydrate − 3.56g anhydrate

1.16g H2OStep 3: 3.56g CaCl2 1mol CaCl2

110.98g CaCl2 1.16g H2O 1mol H2O

18.02g H2O

Calcium chloride: Ca2+ Cl−

CaCl2

= 0.0321 molCaCl2

= 0.0644 mol H2O

SIG FIGS IN ANSWER MUST MATCH GIVEN!!

A calcium chloride hydrate has a mass of 4.72g. After heater for several minutes the mass of the anhydrate is found to be 3.56g. Determine the

formula and name of the hydrate.

Step 4: Ratio 0.0644 mol H2O 20.0321 mol CaCl2

Step 5: CaCl2 2H2O

Step 6: calcium chloride dihydrate

≈ 1

Example 2What is the formula an name for a hydrate which consists of 76.9% CaSO3 and 23.1% H2O?

SIG FIGS IN ANSWER MUST MATCH GIVEN!!

Step 3: 76.9g CaSO3 1mol CaSO3110.98g CaSO3

23.1g H2O 1mol H2O 18.02g H2O

= 0.640 mol CaSO3

= 1.28 mol H2O

Step 4: Ratio 1.28 mol H2O 20.640 mol CaSO3

Step 5: CaSO3 2H2O

Step 6: calcium sulfite dihydrate

≈ 1