The Hydrates Some Ionic Compounds Have a Number of Water Molecules

Special Ionic Compounds – The Hydrates

Some ionic compounds have a number of water molecules hanging around them. These special compounds are called HYDRATES.

A hydrate name or formula has two parts – the first part is the ionic compound, the second part is the water. We separate the two parts with a giant asterisk ( *) or dot ( ·)

CuSO4 * 5 H2O is named in two steps:

The ionic compound is named the regular way, name the cation, name the anion.

Next use your molecular prefixes (mono, di, tri etc.) in front of the word hydrate to denote the number of water molecules.

Answer: copper (II) sulfate pentahydrate

Coming up with the formula for magnesium sulfate heptahydrate is also done in two steps:

First come up with the formula for the ionic compound the usual way, by balancing the charges on the ions.

Check the prefix to find out how many water (H2O) molecules are with the ionic compound.

2+ 2- Mg with SO4 is MgSO4 and heptahydrate means seven water.

Answer: MgSO4 · 7H2O

Nomenclature of Acids

Acids are hydrogen compounds that are dissolved in water. Technically they are molecular, but unlike molecular compounds they will conduct electricity in water. The formula of an acid is always followed by (aq) which means in aqueous solution, in other words, dissolved in water.

. Anything dissolved in water has (aq) after its formula. All acids will have it, but just because there is (aq) does NOT mean it is an acid. It is only an acid if it is a hydrogen compound in water.

Hydrogen compounds are molecular but they don’t use prefixes and we use the hydrogen to balance out charges with an anion. The hydrogen is considered to have a +1 charge. Weird, huh?

Naming and writing formulas for acids is always a two-step process:

A. Naming Acids: 1. Look at the formula and name the acid as a hydrogen compound. It will be named hydrogen ______. The blank represents the name of some sort of ion

2. The names of hydrogen compounds can only have one of three possible endings:

hydrogen _____ide

hydrogen _____ate

hydrogen _____ite

The name of the acid depends on which ion ending you have in your formula:

hydrogen _____ide becomes hydro _____ic acid

hydrogen _____ate becomes ______ic acid

hydrogen _____ite becomes ______ous acid

Examples:

HF (aq) is hydrogen fluoride , so this is hydrofluoric acid

HClO4 (aq) is hydrogen perchlorate , so it would be called perchloric acid

HNO2 (aq) is hydrogen nitrite , so it would be called nitrous acid

B. Writing formulas for acids: 1. Look at the name and work backwards to change it into a hydrogen compound:

hydro____ic acid would be hydrogen _____ide ______ic acid would be hydrogen ______ate ______ous acid would be hydrogen ______ite

2. Make your formula by balancing charges. The hydrogen is always H+, and the anions are on your periodic table or in the polyatomic ion chart on the back.

hydrochloric acid is hydrogen chloride. The hydrogen is H+ and the chloride is Cl- so the formula is HCl (aq)

+ - nitric acid is hydrogen nitrate. The hydrogen is H , and nitrate is NO3 so the

formula is HNO3 (aq) + – chlorous acid is hydrogen chlorite. The hydrogen is H and the chlorite is ClO2

so the formula is HClO2 (aq).

Special Situation and Exceptions to Rules:

Any acid that contains the root “sulf” or “phosph”, add an extra syllable to the middle of the acid name.

For “sulf” add ur For “phosph” add or

H2SO4 (aq) is called sulfuric acid, not sulfic acid

H3PO4 (aq) is called phosphoric acid , not phosphic acid

The other difference is for organic acids. They have anions that end in –COO In these cases you still balance the charges with the H+, but you put the H at the end of the formula instead of at the beginning.