Question

Write the name from the formula or the formula from the name for each hydrated ionic compound. \begin{equation} \begin{array}{l}{\text { a. } \mathrm{CoSO}_{4} \cdot 7 \mathrm{H}_{2} \mathrm{O}} \\ {\text { b. iridium (III) bromide tetrahydrate }} \\ {\text { c. } \mathrm{Mg}\left(\mathrm{BrO}_{3}\right)_{2} \cdot 6 \mathrm{H}_{2} \mathrm{O}} \\ {\text { d. potassium carbonate dihvdrate }}\end{array} \end{equation}

Short answer

a. Cobalt(II) sulfate heptahydrate, b. IrBr3·4H2O, c. Magnesium bromate hexahydrate, d. K2CO3·2H2O

Step-by-step solution

Identifying the Formula and Naming It

For part a, we identify the chemical compound as cobalt(II) sulfate heptahydrate. This is based on the ionic formula where Co is cobalt with a +2 charge, SO4 is sulfate, and the ·7H2O indicates seven water molecules are associated with each formula unit of the compound.

Writing the Formula from the Name

For part b, iridium(III) bromide tetrahydrate is an ionic compound with iridium having a +3 charge. The bromide indicates the presence of Br with a -1 charge each. Tetrahydrate suggests that there are four water molecules, hence the formula is IrBr3·4H2O.

Identifying the Formula and Naming It

For part c, the formula Mg(BrO3)2·6H2O represents magnesium bromate hexahydrate. Mg is magnesium, BrO3 is bromate, and the ·6H2O means there are six water molecules per formula unit.

Writing the Formula from the Name

For part d, potassium carbonate dihydrate has the formula K2CO3·2H2O. Potassium carbonate is K2CO3 and 'dihydrate' specifies that there are two water molecules associated with each formula unit of the salt.

Key concepts

Chemical Formula Naming

Understanding how to name chemical formulas is a fundamental skill in chemistry. It is the way chemists communicate the exact composition of a compound. For ionic compounds, such as salts, the cation (positively charged ion) is named first, followed by the anion (negatively charged ion). In the case of transition metals that can have multiple charges, the charge is indicated in parentheses using Roman numerals. For example, cobalt can have a +2 or +3 charge, so it is written as cobalt(II) or cobalt(III) respectively.

When naming hydrated ionic compounds, the prefix 'hydrate' is used with a Greek prefix to indicate the number of water molecules. These prefixes range from 'mono-' for one, to 'deca-' for ten. Hence, 'heptahydrate' corresponds to seven water molecules being present, as in the cobalt(II) sulfate example (CoSO₄·7H₂O).

Ionic Compound Hydration

Hydration refers to the incorporation of water molecules into an ionic compound's crystal structure. When ionic compounds are formed, water molecules can be trapped and held by electrostatic forces between the ions. This process creates what we know as hydrated ionic compounds. Hydration greatly affects the physical properties of the compounds, such as their solubility, melting point, and structure. Chemists express hydration by appending 'hydrate' with the appropriate prefix, as seen in the step by step solution provided for the exercise.

Naming Hydrated Salts

When it comes to naming hydrated salts, a systematic approach is used that takes into account both the salt and the level of hydration. The rule is simple: First, name the ionic compound (considering any transition metal charges), then add the term for water of hydration. For example, in the case of magnesium bromate with six water molecules, it is named magnesium bromate hexahydrate. The beauty of this naming scheme is that it provides clear communication regarding not only the components of the salt but also the precise amount of water associated with the compound.

A mnemonic device to remember the Greek prefixes might be helpful: 'mon' for one, 'di' for two, 'tri' for three, and so on. Keeping these prefix names in mind can make naming hydrated salts more intuitive.

Identifying Chemical Formulas

The ability to identify chemical formulas is just as important as naming them. Identifying involves looking at a written formula and understanding which elements and how many of each are present in the compound, including the water of hydration. This forms the basis for understanding the compound's properties and behavior. Each element in the formula is represented by its chemical symbol, and the amount of each element is indicated by subscript numbers. If the compound is hydrated, the number of water molecules is represented by a coefficient followed by H₂O.

Consistent practice in identifying chemical formulas from names and writing chemical names from formulas builds a strong foundation in chemical literacy. This skill enables students to engage more deeply with the material in a chemical context.