NAPLEX (North American Pharmacist Licensure Examination) Practice Exam

How many mEq/mL does a 10% solution of KCl contain?

1. 20 mEq/10 mL

2. 10 mEq/10 mL

3. 20 mEq/15 mL

4. 15 mEq/20 mL

The correct answer is: 20 mEq/15 mL

A 10% solution of KCl means that there are 10 grams of KCl in 100 mL of solution. To determine the mEq/mL, we need to consider the molar mass and valence of KCl. The molar mass of KCl is approximately 74.5 g/mol and KCl dissociates into K+ and Cl- ions in solution, with each K+ or Cl- ion carrying 1 equivalent of charge. Therefore, in 74.5 g of KCl, there is 1 mole of KCl, which is equivalent to 1 equivalent (1 mole) of K+ ions and 1 equivalent (1 mole) of Cl- ions. Now, if 10% of the solution is KCl, there are 10 grams of KCl in 100 mL of solution. This corresponds to 10/74.5 moles of KCl. As each mole of KCl produces 1 mole of K+ ions, we have 10/74.5 moles of K+ ions in 100 mL of the solution. To determine the mEq/mL, we convert moles of K+ ions to milliequivalents (mEq) by multiplying by the valence of the ion. Since K+ has a valence of 1, the mEq/mL of a 10% solution of KCl is 10/74.5 * 1 = 0.134 mEq/mL. Consequently, the correct answer is 20 mEq/15 mL since 0.134 mEq/mL multiplied by 15 mL gives us 2 mEq of K+ ions per 15 mL of the solution, which simplifies to 20 mEq per 15 mL.