7.4: Names and Formulas of Bases


Bases

A base can be simply defined as an ionic compound that produces hydroxide ions when dissolved in water. One of the most commonly used bases is sodium hydroxide, illustrated below.

Some bases do not contain hydroxide, but they can react with water, accepting an H+, and producing OH-. Ammonia is an example of such a base:

NH3(g) + H2O(l) → NH4+(aq) + OH-(aq)

Names and Formulas of Bases

There is no special system for naming bases. Since they all contain the $$\ce{OH^-}$$ anion, names of bases end in hydroxide. The cation is simply named first. Some examples of names and formulas for bases are shown in the table below.

 Table $$\PageIndex{1}$$ Formula Name $$\ce{NaOH}$$ sodium hydroxide $$\ce{Ca(OH)_2}$$ calcium hydroxide $$\ce{NH_4OH}$$ ammonium hydroxide NH3 ammonia Fe(OH)2 Iron(II) hydroxide or Ferrous hydroxide Fe(OH)3 Iron(III) hydroxide or Ferric hydroxide

Notice that because hydroxides are ionic compounds, the number of hydroxides in the formula does not affect the name. The compound must be neutral, so the charges of the ions are balanced just as for other ionic compounds. The sodium ion $$\left( \ce{Na^+} \right)$$ requires one $$\ce{OH^-}$$ ion to balance the charge, so the formula is $$\ce{NaOH}$$. Calcium $$\left( \ce{Ca^{2+}} \right)$$ requires two $$\ce{OH^-}$$ ions to balance the charge, so the formula is $$\ce{Ca(OH)_2}$$. Hydroxide ion is a polyatomic ion, and must be in parentheses when there is more than one in a formula.

Concentrated aqueous solutions of ammonia (NH3) contain significant amounts of the hydroxide ion, even though the dissolved substance is not primarily ammonium hydroxide (NH4OH) as is often stated on the label. Thus aqueous ammonia solution is also a common base.

For transition metals, more than one cation is possible. In this case, the name of the hydroxide includes the charge of the cation in roman numerals and parenthesis. For example, iron can form two possible cattions: Fe2+ or Fe3+, and therefore, two hydroxides are possible: iron(II) hydroxide with formula Fe(OH)2, or  iron(III) hydroxide with formula Fe(OH)3. Alternatively, a classic nomenclature system uses the root name of the metal in Latin plus the ending -ous to refer to the lowest possible charge, and the  -ic  ending to refer to the highest possible charge. In this system, Fe(OH)2 and Fe(OH)3 are named Ferrous hydroxide and Ferric hydroxide, respectively.

Summary

• Bases are ionic compounds that produce hydroxide ions when dissolved in water.
• The cation is named first followed by hydroxide. When more than one cation is possible, the charge of the ion in roman numerals must be included.