Problem 23 - Chemistry An Atoms First Approach

Question

For an acid or a base, when is the normality of a solution equal to the molarity of the solution and when are the two concentration units different?

Step-by-Step Solution

Answer

In summary, normality and molarity are equal for acids and bases when they donate or accept only one hydrogen ion or hydroxide ion (n = 1), such as in the case of hydrochloric acid (HCl) and sodium hydroxide (NaOH). The two concentration units are different when the compound donates or accepts more than one hydrogen ion or hydroxide ion, with normality being equal to the molarity multiplied by the number of reactive ions (n), as seen in sulfuric acid (H₂SO₄) and calcium hydroxide (Ca(OH)₂).

Step 1: Case 1: Normality equals Molarity for acids and bases

Normality and molarity will be equal for acids and bases when n = 1. That means, when the acidic or basic substance donates or accepts only one hydrogen ion or hydroxide ion in the reaction, the two concentration units will be the same. Examples of such compounds include: 1. Hydrochloric acid (HCl): When dissolved in water, HCl donates one hydrogen ion (H⁺) and forms the chloride ion (Cl⁻). 2. Sodium hydroxide (NaOH): When dissolved in water, NaOH dissociates into one sodium ion (Na⁺) and one hydroxide ion (OH⁻).

Step 2: Case 2: Normality is not equal to Molarity for acids and bases

Normality and molarity will be different for acids and bases when n is not equal to 1. That means, when the acidic or basic substance donates or accepts more than one hydrogen ion or hydroxide ion in the reaction, the two concentration units will be different. Examples of such compounds include: 1. Sulfuric acid (H₂SO₄): When H₂SO₄ dissolves in water, it donates two hydrogen ions (H⁺) and forms the sulfate ion (SO₄²⁻). In this case, n = 2, so normality = 2 × molarity. 2. Calcium hydroxide (Ca(OH)₂): When Ca(OH)₂ dissolves in water, it dissociates into one calcium ion (Ca²⁺) and two hydroxide ions (OH⁻). In this case, n = 2, so normality = 2 × molarity. In summary, normality and molarity are equal for acids and bases when they donate or accept only one hydrogen ion or hydroxide ion (n = 1). The two concentration units are different when the compound donates or accepts more than one hydrogen ion or hydroxide ion, with normality being equal to the molarity multiplied by the number of reactive ions (n).