## MCAT General Chemistry Review - Alexander Stone Macnow, MD 2019-2020

# Acids and Bases

Polyvalence and Normality

LEARNING GOALS

After Chapter 10.3, you will be able to:

· Describe how equivalents of acid or base are calculated

· Calculate the normality of a solution given its formula and molarity

The relative acidity or basicity of an aqueous solution is determined by the relative concentrations of acid and base equivalents. An **acid equivalent** is equal to one mole of H^{+} (or, more properly, H_{3}O^{+}) ions; a **base equivalent** is equal to one mole of OH^{−} ions. Some acids and bases are **polyvalent**; that is, each mole of the acid or base liberates more than one acid or base equivalent. Under the Brønsted—Lowry definition, such acids or bases could also be termed **polyprotic**. For example, the divalent diprotic acid H_{2}SO_{4} undergoes the following dissociation in water:

One mole of H_{2}SO_{4} produces two acid equivalents (2 moles of H_{3}O^{+}). Notice that the first dissociation goes to completion, but the second dissociation reaches an equilibrium state. The acidity or basicity of a solution depends on the concentration of acidic or basic equivalents that can be liberated. The quantity of acidic or basic capacity is directly indicated by the solution’s **normality**, described in Chapter 9 of *MCAT General Chemistry Review*. For example, each mole of H_{3}PO_{4} yields three moles (equivalents) of H_{3}O^{+}. Therefore, a 2 *M* H_{3}PO_{4} solution would be 6 *N*.

Another measurement useful for acid—base chemistry is **gram equivalent weight**. Chapter 4 of *MCAT General Chemistry Review* defined and discussed this term extensively. The gram equivalent weight is the mass of a compound that produces one equivalent (one mole of charge). For example, H_{2}SO_{4} (molar mass: ) is a divalent acid, so each mole of the acid compound yields two acid equivalents. The gram equivalent weight is 98 ÷ 2 = 49 grams. That is, the complete dissociation of 49 grams of H_{2}SO_{4} will yield one acid equivalent (one mole of H_{3}O^{+}). Common polyvalent acids include H_{2}SO_{4}, H_{3}PO_{4}, and H_{2}CO_{3}. Common polyvalent bases include Al(OH)_{3}, Ca(OH)_{2}, and Mg(OH)_{2}.

**Bridge**

To review normality in more detail, revisit the calculations performed in Chapter 4 of *MCAT General Chemistry Review*. These are critical calculations for polyvalent acids and bases.

**MCAT Concept Check 10.3:**

Before you move on, assess your understanding of the material with these questions.

1. What species are considered the equivalents for acids and bases, respectively?

o Acids:

o Bases:

2. Calculate the normality of the following solutions:

o 2 *M* Al(OH)_{3}:

o 16 *M* H_{2}SO_{4}: