The Nomeclature of Hydrates

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Hydrates are substances that include water in their formula. The water is not actually part of the chemical substance and this is reflected in the way the formula is written.

Here is an example: CuSO4 5H2O

This formula means that for every one CuSO4 in the piece of this substance you are holding, there are also five water molecules. No, the substance is not wet, it appears dry. There are some hydrates that have a wet appearance, but most appear perfectly dry to the eye and to the touch.

The dot IS NOT a multiplication sign. Remember, this is chemistry, not math.

Here is the name: copper(II) sulfate pentahydrate. Notice penta- meaning five and -hydrate meaning water. You would use this name both when writing the name or speaking it. Don't say something like "copper two sulfate five waters." Say "copper two sulfate pentahydrate."

When you hear "pentahydrate," you have to know to write the dot and then the 5H2O.

Here are the first ten Greek number prefixes:

Number  Prefix  Number  Prefix
one  mono-  six  hexa-
two  di-  seven  hepta-
three  tri-  eight  octa-
four  tetra-  nine  nona-
five  penta-  ten  deca-

Problem #1: Suppose you heard "trihydrate." What would you write?


Problem #2: Suppose you heard "octahydrate." What would you write?


Problem #3: Name this substance: MgSO4 H2O

magnesium sulfate monohydrate

When there is only one water, make sure to still use the proper prefix. Magnesium sulfate hydrate is not the correct name for MgSO4 H2O.

By the way, multiple hydrates exist for MgSO4. In addition to the monohydrate, there also exists the tetrahydrate, pentahydrate, hexahydrate and heptahydrate.

Problem #4: Write the formula for barium chloride dihydrate

BaCl2 2H2O

Problem #5: How many water molecules are associated with one formula unit of Na2SO3 7H2O? Write the name of this compound.


Sodium sulfite heptahydrate.

Problem #6: The compound beryllium sulfate forms a hydrate with four water molecules per formula unit. Write the full name of hydrate and the formula for the hydrate.

beryllium sulfate tetrahydrate

BeSO4 4H2O

Problem #7: There is a hydrate whose name is potassium carbonate sesquihydrate. What is its formula?

Sesqui- is a seldom used Greek prefix. It means one-and-one-half. The formula is:

K2CO3 32H2O

Often, the formula is seen like this:

2K2CO3 3H2O

Sometimes, a decimal is used:

K2CO3 1.5H2O

The 150-year anniversary of something is referred to as the sesquicentennial.

Problem #8: There is a hydrate whose name is calcium sulfate hemihydrate. What is its formula?

Hemi- is a seldom used Greek prefix (but used more often than sesqui-). It means one-half. The formula is:

CaSO4 12H2O

Often, the formula is seen like this:

2CaSO4 H2O

Sometimes, a decimal is used:

CaSO4 0.5H2O

By the way, semi- is the Latin prefix for one-half and demi- is the French prefix for one-half. All three prefixes are used in the word hemidemisemiquaver, which is a 1/64 note in music (a quaver being a 1/8 note).

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