Question 6.14: A typical concentration of calcium ion in blood plasma is 4 ...

Step 1. The calcium ion has a 2+ charge (recall that calcium is in Group IIA of the periodic table; hence, a 2+ charge on the calcium ion).

Step 2. We will need three conversion factors:
meq (milliequivalents) → eq (equivalents)
eq (equivalents) → moles of charge
moles of charge → moles of calcium ion

Step 3. Using dimensional analysis as in Example 6.10,
\pmb{ \frac{4  \cancel{meq  Ca^{2+}}}{1  L} \times \frac{1  \cancel{eq  Ca^{2+}}}{10^3  \cancel{meq  Ca^{2+}}} \times \frac{1  \cancel{mol  charge}}{1  \cancel{eq  Ca^{2+}}} \times \frac{1  mol  Ca^{2+}}{2  \cancel{mol  charge} } = \frac{2 \times 10^{-3 }  mol  Ca^{2+}}{L} }