Question

The most acidic of the following \(0.10 \mathrm{M}\) salt solutions is (a) \(\mathrm{Na}_{2} \mathrm{S} ;\) (b) \(\mathrm{NaHSO}_{4} ;\) (c) \(\mathrm{NaHCO}_{3} ;\) (d) \(\mathrm{Na}_{2} \mathrm{HPO}_{4}\)

Short answer

The most acidic 0.10 M salt solution among the given solutions is \(\mathrm{NaHSO}_{4}\) because \(\mathrm{HSO}_{4}^{-}\), the anion present in this salt, has the parent acid \(\mathrm{H}_{2}\mathrm{SO}_{4}\), which is the strongest compared to the parent acids of the anions in other salts.

Step-by-step solution

Identify the Anions and Their Parent Acids

Firstly, identify the anions in each salt and their parent acids: In (a) \(\mathrm{Na}_{2} \mathrm{S}\), the anion is \(\mathrm{S}^{^{2-}}\) and the parent acid is \(\mathrm{H}_{2} \mathrm{S}\). In (b) \(\mathrm{NaHSO}_{4}\), the anion is \(\mathrm{HSO}_{4}^-\) and the parent acid is \(\mathrm{H}_{2}\mathrm{SO}_{4}\). In (c) \(\mathrm{NaHCO}_{3}\), the anion is \(\mathrm{HCO}_{3}^-\) and the parent acid is \(\mathrm{H}_{2}\mathrm{CO}_{3}\). In (d) \(\mathrm{Na}_{2} \mathrm{HPO}_{4}\), the anion is \(\mathrm{HPO}_{4}^{^{2-}}\) and the parent acid is \(\mathrm{H}_{2}\mathrm{PO}_{4}\).

Determine the Strength of the Parent Acids

Now, the more acidic a solution, the lower its pH value. Therefore, the strength of the parent acid of each anion will determine how acidic the salt solution is: \(\mathrm{H}_{2}\mathrm{S}\) is a weak acid; \(\mathrm{H}_{2}\mathrm{SO}_{4}\) is a strong acid; \(\mathrm{H}_{2}\mathrm{CO}_{3}\) is a weak acid; and \(\mathrm{H}_{2}\mathrm{PO}_{4}\) is a moderate acid.

Find the Most Acidic Solution

Since the most acidic solution will be the one whose anion's parent acid is the strongest, and given that \(\mathrm{H}_{2}\mathrm{SO}_{4}\) is the strongest among the identified parent acids, then the solution \(\mathrm{NaHSO}_{4}\), where \(\mathrm{HSO}_{4}^{-}\) is the anion, will be the most acidic.

Key concepts

Anions

Anions are negatively charged ions formed when an atom gains one or more electrons. They are part of the salts that can influence the acidity or basicity of a solution. In the context of the salts given in the problem, the anion determines the property of the solution in terms of acidity. Let's examine the anions in the examples:

• Na₂S: The anion is S²⁻. It originates from the dissociation of H₂S.
• NaHSO₄: The anion here is HSO₄⁻, which comes from H₂SO₄.
• NaHCO₃: This salt contains the anion HCO₃⁻, derived from H₂CO₃.
• Na₂HPO₄: The anion HPO₄²⁻ comes from H₂PO₄⁻.

Understanding the anions in each salt helps us determine which parent acid they are associated with, and this further helps predict the salt's behavior in a solution.

Parent Acids

Parent acids are the original acids that provide the anion part in the salt. Knowing the parent acid is crucial because it influences the potential acidity when the salt is dissolved in water. Generally, the stronger the parent acid, the more acidic the resulting solution can be.
Let's look at the parent acids for each salt in our example:

• For S²⁻ in Na₂S, the parent acid is H₂S, known for being a weak acid.
• HSO₄⁻ in NaHSO₄ has H₂SO₄ as its parent acid. H₂SO₄ is a strong acid, which often leads to a more acidic solution.
• The anion HCO₃⁻ in NaHCO₃ comes from H₂CO₃, a weak acid.
• HPO₄²⁻ in Na₂HPO₄ is associated with the moderate acid H₂PO₄⁻.

Observing the strength of these parent acids helps us assess which salt solution will be most acidic.

Solution pH

The pH of a solution is a measure of its acidity or basicity. It is important as it can affect chemical reactions and the behavior of substances when they dissolve in liquid. A lower pH indicates a higher acidity, while a higher pH suggests more basic characteristics.
When salts dissolve in water, they can affect the pH by either contributing H⁺ ions (increase acidity) or OH⁻ ions (increase basicity). For the salts given in the problem, the solution's pH is influenced by how much the anion from each salt can contribute to the H⁺ concentration once dissolved.

• NaHSO₄: The HSO₄⁻ anion can release H⁺ ions into the solution, making it very acidic.
• Na₂S: The presence of S²⁻ doesn't contribute significantly to increasing acidity, resulting in a less acidic solution.
• NaHCO₃: As a weak acid constituent, HCO₃⁻ generally leads to a mild acidic or even slightly basic solution.
• Na₂HPO₄: With HPO₄²⁻, the solution tends towards a more neutral or slightly basic pH due to it acting like a weak base.

Thus, understanding how each anion affects the solution's pH helps us identify why NaHSO₄ is the most acidic. This insight grasps the basics of salt solutions and their intriguing influence on pH.