Molar Mass and Percent Composition Calculator

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molar mass of hydrates copper sulfate pentahydrate

Molar mass of hydrates: CuSO4.5H2O, MgSO4.7H2O and crystal water

In a hydrate the dot separates the salt from the water molecules held in the crystal, and the molar mass adds the two parts. The tool reads CuSO4.5H2O, CuSO4·5H2O and CuSO4*5H2O as the same formula.

Adding the salt and the crystal water

The molar mass of a hydrate is that of the anhydrous salt plus n times that of water. In copper(II) sulfate pentahydrate, CuSO4.5H2O, CuSO4 weighs 63.546 + 32.06 + 4 x 15.999 = 159.602 g/mol and the five waters weigh 5 x 18.015 = 90.075 g/mol, giving 249.677 g/mol. The percentage of water is 90.075 / 249.677 = 36.08%. In other common hydrates: Epsom salt, MgSO4.7H2O, is 246.466 g/mol and 51.2% water; sodium sulfate decahydrate, Na2SO4.10H2O, is 322.186 g/mol and 55.9% water; gypsum, CaSO4.2H2O, is 172.164 g/mol and 20.9%; ferrous sulfate, FeSO4.7H2O, is 278.006 g/mol.
Crystal water has direct use in the lab. Heating 10.00 g of CuSO4.5H2O until the salt turns white drives off the water, and the predicted final mass is 10.00 x 159.602 / 249.677 = 6.39 g. If the weighing gives 6.6 g, the salt still held water. When preparing solutions, whoever uses a hydrate must weigh more: for 0.100 mol of CuSO4 that is 24.97 g of the pentahydrate and only 15.96 g of anhydrous salt. Mixing the two up is a classic concentration error, 56% more or less mass.
Some hydrates carry half a water molecule per formula unit. Plaster of Paris, calcium sulfate hemihydrate, is written CaSO4.0.5H2O or CaSO4·½H2O, and the tool accepts both spellings: 136.134 + 0.5 x 18.015 = 145.142 g/mol. Cobalt(II) chloride hydrate, CoCl2.6H2O, is 237.923 g/mol and 45.4% water, and changes from blue to pink as it absorbs moisture. Only the formula shown matters for the calculation: the tool cannot tell a real hydrate from a typed formula, so check n on the bottle label.

Hydrates split into salt and water

Copper sulfate pentahydrate

Input: CuSO4.5H2O
Expected output: CuSO4 + 5H2O

249.677 g/mol; water 36.08%.

Epsom salt

Input: MgSO4.7H2O
Expected output: MgSO4 + 7H2O

246.466 g/mol; 7 waters weigh 126.105.

Sodium sulfate decahydrate

Input: Na2SO4.10H2O
Expected output: Na2SO4 + 10H2O

322.186 g/mol; water 55.9%.

Gypsum

Input: CaSO4.2H2O
Expected output: CaSO4 + 2H2O

172.164 g/mol; water 20.9%.

Cobalt chloride hexahydrate

Input: CoCl2.6H2O
Expected output: CoCl2 + 6H2O

237.923 g/mol; moisture indicator.

Plaster of Paris, half water

Input: CaSO4.0.5H2O
Expected output: CaSO4 + 0.5H2O

145.142 g/mol; accepts 0.5 or ½.

Full tool FAQ

How do you calculate the molar mass of a compound?

Add the atomic weights of every atom in the formula. For aluminum sulfate, Al2(SO4)3: 2 Al + 3 S + 12 O = 2 x 26.9815 + 3 x 32.06 + 12 x 15.999 = 342.131 g/mol. A subscript outside parentheses multiplies everything inside them.

What is the difference between molar mass, molecular mass and molecular weight?

Why does the result differ from my textbook?

How do you calculate the molar mass of a hydrate?

How do you calculate percent composition?

How do you convert grams to moles and molecules?

How do you find the empirical formula?

Does the tool accept ions and isotopes?

Where do the atomic weights come from, and do they change?

Why do some elements show an asterisk or a dagger?

Frequently asked questions

Is the dot in the middle of the formula a multiplication?

No. It joins two parts that exist together in the crystal, the salt and the water. The number after the dot multiplies only the part that follows: in CuSO4.5H2O, the 5 applies to H2O and CuSO4 keeps 1.

How do I compute the percentage of water in a hydrate?

Divide the mass of the n waters by the molar mass of the hydrate and multiply by 100. For MgSO4.7H2O, 126.105 / 246.466 x 100 = 51.2%. The result tells how much mass is lost when heating to the anhydrous salt.

What changes in the Rounded mode?

Water becomes 18 and CuSO4.5H2O comes to 249.5 g/mol, with Cu = 63.5, S = 32 and O = 16. The water percentage becomes 36.07%. Use the mode your exam or report adopts.

Next steps

Contribution of each element to the molar mass
Element	Atoms	Atomic mass	Subtotal (g/mol)	Mass %
C	6	12.011*	72.066	40.00%
H	12	1.008*	12.096	6.71%
O	6	15.999*	95.994	53.28%
Molar mass	180.156	100%

Contribution of each element to the molar mass

Element

Atoms

Atomic mass

Subtotal (g/mol)

Mass %

12.011*

72.066

40.00%

1.008*

12.096

6.71%

15.999*

95.994

53.28%

Molar mass

180.156

100%

Adding the salt and the crystal water

The molar mass of a hydrate is that of the anhydrous salt plus n times that of water. In copper(II) sulfate pentahydrate, CuSO4.5H2O, CuSO4 weighs 63.546 + 32.06 + 4 x 15.999 = 159.602 g/mol and the five waters weigh 5 x 18.015 = 90.075 g/mol, giving 249.677 g/mol. The percentage of water is 90.075 / 249.677 = 36.08%. In other common hydrates: Epsom salt, MgSO4.7H2O, is 246.466 g/mol and 51.2% water; sodium sulfate decahydrate, Na2SO4.10H2O, is 322.186 g/mol and 55.9% water; gypsum, CaSO4.2H2O, is 172.164 g/mol and 20.9%; ferrous sulfate, FeSO4.7H2O, is 278.006 g/mol.

Crystal water has direct use in the lab. Heating 10.00 g of CuSO4.5H2O until the salt turns white drives off the water, and the predicted final mass is 10.00 x 159.602 / 249.677 = 6.39 g. If the weighing gives 6.6 g, the salt still held water. When preparing solutions, whoever uses a hydrate must weigh more: for 0.100 mol of CuSO4 that is 24.97 g of the pentahydrate and only 15.96 g of anhydrous salt. Mixing the two up is a classic concentration error, 56% more or less mass.

Some hydrates carry half a water molecule per formula unit. Plaster of Paris, calcium sulfate hemihydrate, is written CaSO4.0.5H2O or CaSO4·½H2O, and the tool accepts both spellings: 136.134 + 0.5 x 18.015 = 145.142 g/mol. Cobalt(II) chloride hydrate, CoCl2.6H2O, is 237.923 g/mol and 45.4% water, and changes from blue to pink as it absorbs moisture. Only the formula shown matters for the calculation: the tool cannot tell a real hydrate from a typed formula, so check n on the bottle label.

Hydrates split into salt and water

Copper sulfate pentahydrate

Input: CuSO4.5H2O
Expected output: CuSO4 + 5H2O

249.677 g/mol; water 36.08%.

Epsom salt

Input: MgSO4.7H2O
Expected output: MgSO4 + 7H2O

246.466 g/mol; 7 waters weigh 126.105.

Sodium sulfate decahydrate

Input: Na2SO4.10H2O
Expected output: Na2SO4 + 10H2O

322.186 g/mol; water 55.9%.

Gypsum

Input: CaSO4.2H2O
Expected output: CaSO4 + 2H2O

172.164 g/mol; water 20.9%.

Cobalt chloride hexahydrate

Input: CoCl2.6H2O
Expected output: CoCl2 + 6H2O

237.923 g/mol; moisture indicator.

Plaster of Paris, half water

Input: CaSO4.0.5H2O
Expected output: CaSO4 + 0.5H2O

145.142 g/mol; accepts 0.5 or ½.

Full tool FAQ

How do you calculate the molar mass of a compound?

What is the difference between molar mass, molecular mass and molecular weight?

Why does the result differ from my textbook?

How do you calculate the molar mass of a hydrate?

How do you calculate percent composition?

How do you convert grams to moles and molecules?

How do you find the empirical formula?

Does the tool accept ions and isotopes?

Where do the atomic weights come from, and do they change?

Why do some elements show an asterisk or a dagger?

Frequently asked questions

Is the dot in the middle of the formula a multiplication?

How do I compute the percentage of water in a hydrate?

What changes in the Rounded mode?

Water becomes 18 and CuSO4.5H2O comes to 249.5 g/mol, with Cu = 63.5, S = 32 and O = 16. The water percentage becomes 36.07%. Use the mode your exam or report adopts.