Question 6.10: Calculate the number of equivalents per liter (eq/L) of phos...

Step 1. It is necessary to use two conversion factors:
mol PO_4  ^{3–} → mol charge
and
mol charge → eq PO_4  ^{3–}

Step 2. Arranging these factors in sequence yields:
\frac{5.0 \times 10^{-3}  \cancel{mol  PO_4  ^{3–}}}{1  L} \times \frac{3  \cancel{mol  charge}}{1  \cancel{mol  PO_4  ^{3–}}} \times \frac{1  eq  PO_4  ^{3–}}{1  \cancel{mol  charge}} = \frac{1.5 \times 10^{-2}  eq  PO_4  ^{3–}}{L}