Formula Writing Chemistry Example 2

Solution

1. 1
   Crisscross method: take the magnitude of each ion's charge and use it as the subscript for the other ion.
2. 2
   $\text{Ca}^{2+}$ and $\text{PO}_4^{3-}$: the subscript for Ca becomes 3 (from the $3-$ charge) and the subscript for $\text{PO}_4$ becomes 2 (from the $2+$ charge).
3. 3
   Formula: $\text{Ca}_3(\text{PO}_4)_2$. Verify: $3(+2) + 2(-3) = +6 - 6 = 0$. Neutral. Parentheses are needed around $\text{PO}_4$ because its subscript is greater than 1.