Question

an ionic compound is named cobalt (iii) phosphide. complete the table to determine the number of ions of each element in one formula unit of the compound. enter a negative sign before negative numbers. positive numbers do not need a positive sign. in one formula unit what is the formula for the ionic compound? element: cobalt (iii), phosphorus; charge of one ion; total charge of ion(s) in compound; number of ion(s) in compound*

Explanation:

Step1: Determine ion charges

Cobalt(III) ion: $\text{Co}^{3+}$ (charge +3). Phosphide ion: $\text{P}^{3-}$ (charge -3).

Step2: Balance charges

To balance total charge (0), number of $\text{Co}^{3+}$ and $\text{P}^{3-}$ ions: Let $x$ = Co ions, $y$ = P ions. $3x - 3y = 0 \implies x = y$. So 1 Co and 1 P.

Step3: Write formula

Combine ions: $\text{CoP}$.

Answer:

The formula for cobalt(III) phosphide is $\boldsymbol{\text{CoP}}$.

• Cobalt(III) charge: $+3$; Total charge from Co: $+3$; Number of Co ions: $1$
• Phosphide charge: $-3$; Total charge from P: $-3$; Number of P ions: $1$