Example 2.11: Predicting the Formula of a Compound with a Polyatomic Anion
Baking powder contains calcium dihydrogen phosphate, an ionic compound composed of the ions
\(\ce{Ca^2+}\) and
\(\ce{H2PO4-}\). What is the formula of this compound?
Solution
The positive and negative charges must balance, and this ionic compound must be electrically neutral. Thus, we must have two negative charges to balance the
\(\ce{2+}\) charge of the calcium ion. This requires a ratio of one
\(\ce{Ca^2+}\) ion to two
\(\ce{H2PO4-}\) ions. We designate this by enclosing the formula for the dihydrogen phosphate ion in parentheses and adding a subscript
2. The formula and dissociation equation is
\(\ce{Ca(H2PO4)2}\)\(\ce{->}\)\(\ce{Ca^2+}\)\(\ce{ + }\)\(\ce{2 H2PO4-}\)\(\ce{ }\)
Inspection or adding up the charges shows that with this stoichiometry, the ionic compound is indeed net neutral.
\(ν_{\mathrm{\ce{Ca^2+}}}\) \(= 1\)
\(ν_{\mathrm{\ce{H2PO4^2-}}}\) \(= 2\)
\(\mathrm{Charge}_{\mathrm{\ce{Ca^2+}}}\) \(= 2\)
\(\mathrm{Charge}_{\mathrm{\ce{H2PO4-}}}\) \(= -1\)
\(\mathrm{Charge}_{\mathrm{total}}\) \(= ν_{\mathrm{\ce{Ca^2+}}} \cdot \mathrm{Charge}_{\mathrm{\ce{Ca^2+}}} + ν_{\mathrm{\ce{H2PO4^2-}}} \cdot \mathrm{Charge}_{\mathrm{\ce{H2PO4-}}}\)
\(\ \ \ =1 \cdot 2 + 2 \cdot (-1)\)
\(\ \ \ =2 + -2\)
\(\ \ \ =0\)