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Writing Formulae

6.6 Writing formulae (ESACD)

In Chapter 2 you learnt about the writing of chemical formulae. Table 6.2 shows some of the common anions and cations that you should know.

 Name of compound ion formula Name of compound ion formula Acetate (ethanoate) $$\text{CH}_{3}\text{COO}^{-}$$ Manganate $$\text{MnO}_{4}^{2-}$$ Ammonium $$\text{NH}_{4}^{+}$$ Nitrate $$\text{NO}_{3}^{-}$$ Carbonate $$\text{CO}_{3}^{2-}$$ Nitrite $$\text{NO}_{2}^{-}$$ Chlorate $$\text{ClO}_{3}^{-}$$ Oxalate $$\text{C}_{2}\text{O}{4}^{2-}$$ Chromate $$\text{CrO}_{4}^{-}$$ Oxide $$\text{O}_{2}^{-}$$ Cyanide $$\text{CN}^{-}$$ Permanganate $$\text{MnO}_{4}^{-}$$ Dihydrogen phosphate $$\text{H}_{2}\text{PO}_{4}^{-}$$ Peroxide $$\text{O}_{2}^{2-}$$ Hydrogen carbonate $$\text{HCO}_{3}^{-}$$ Phosphate $$\text{PO}_{4}^{3-}$$ Hydrogen phosphate $$\text{HPO}_{4}^{3-}$$ Phosphide $$\text{P}^{3-}$$ Hydrogen sulphate $$\text{HSO}_{4}^{-}$$ Sulphate $$\text{SO}_{4}^{2-}$$ Hydrogen sulphite $$\text{HSO}_{3}^{-}$$ Sulphide $$\text{S}^{2-}$$ Hydroxide $$\text{OH}^{-}$$ Sulphite $$\text{SO}_{3}^{2-}$$ Hypochlorite $$\text{ClO}^{-}$$ Thiosulphate $$\text{S}_{2}\text{O}_{3}^{2-}$$

Table 6.2: Table showing common compound ions and their formulae

Chemical compounds: names and masses (ESACE)

In Chapter 4 you learnt about atomic masses. In this chapter we have learnt that atoms can combine to form compounds. Molecules are formed when atoms combine through covalent bonding, for example ammonia is a molecule made up of three hydrogen atoms and one nitrogen atom. The relative molecular mass (M) of ammonia $$(\text{NH}_{3})$$ is:

\begin{align*} M & = \text{ relative atomic mass of one nitrogen } + \text{ relative atomic mass of three hydrogens } \\ & = \text{14,0} + 3(\text{1,01}) \\ & = \text{17,03} \end{align*}

One molecule of $$\text{NH}_{3}$$ will have a mass of $$\text{17,03}$$ $$\text{units}$$. When sodium reacts with chlorine to form sodium chloride, we do not get a molecule of sodium chloride, but rather a sodium chloride crystal lattice. Remember that in ionic bonding molecules are not formed. We can also calculate the mass of one unit of such a crystal. We call this a formula unit and the mass is called the formula mass. The formula mass for sodium chloride is:

\begin{align*} M & = \text{ relative atomic mass of one sodium atom } + \text{ relative atomic mass of one chlorine atom } \\ & = \text{23,0} + \text{35,45} \\ & = \text{58,45} \end{align*}

The formula mass for $$\text{NaCl}$$ is $$\text{58,45}$$ $$\text{units}$$.

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Chemical formulae

Exercise 6.5

Write the chemical formulae for each of the following compounds and calculate the relative molecular mass or formula mass:

1. hydrogen cyanide

2. carbon dioxide

3. sodium carbonate

4. ammonium hydroxide

5. barium sulphate

6. copper (II) nitrate

Solution not yet available.

Complete the following table. The cations at the top combine with the anions on the left. The first row is done for you. Also include the names of the compounds formed and the anions.

 $$\text{Na}^{+}$$ $$\text{Mg}^{2+}$$ $$\text{Al}^{3+}$$ $$\text{NH}_{4}^{+}$$ $$\text{H}^{+}$$ $$\text{Br}^{-}$$ name: $$\text{NaBr}$$ $$\text{MgBr}_{2}$$ $$\text{AlBr}_{3}$$ $$(\text{NH}_{4})\text{Br}$$ $$\text{HBr}$$ sodium bromide magnesium bromide aluminium bromide ammonium bromide hydrogen bromide $$\text{S}^{2-}$$ name: $$\text{P}^{3-}$$ name: $$\text{MnO}_{4}^{-}$$ name: $$\text{Cr}_{2}\text{O}_{7}^{2-}$$ name: $$\text{HPO}_{4}^{2-}$$ name:
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