Problem 93 - Chemistry and Chemical Reactivity

Question

For each of the following salts, predict whether a 0.10 M solution has a pH less than, equal to, or greater than 7. (a) \(\mathrm{NaHSO}_{4}\) (b) \(\mathrm{NH}_{4} \mathrm{Br}\) (c) \(\mathrm{KClO}_{4}\) (d) \(\mathrm{Na}_{2} \mathrm{CO}_{3}\) (e) \(\left(\mathrm{NH}_{4}\right)_{2} \mathrm{S}\) (f) \(\mathrm{NaNO}_{3}\) (g) \(\mathrm{Na}_{2} \mathrm{HPO}_{4}\) (h) LiBr (i) \(\mathrm{FeCl}_{3}\) Which solution has the highest pH? The lowest pH?

Step-by-Step Solution

Answer

Na2CO3 has the highest pH; NaHSO4 has the lowest pH.

Step 1: Introduction to Salt Solutions and pH

When salts dissolve in water, they can undergo hydrolysis, which can affect the pH of the solution. To predict the pH of a salt solution, consider the parent acid and base from which the salt is derived. Strong acids/bases form neutral solutions, and weak acids/bases can form acidic or basic solutions.

Step 2: Analyze Each Salt

For each salt, determine if it comes from strong or weak acids and bases: (a) NaHSO4: Comes from strong acid H2SO4 and weak base NaOH. The solution will be acidic (pH < 7). (b) NH4Br: Comes from weak base NH3 and strong acid HBr. The solution will be acidic (pH < 7). (c) KClO4: Comes from strong base KOH and strong acid HClO4. The solution will be neutral (pH = 7). (d) Na2CO3: Comes from strong base NaOH and weak acid H2CO3. The solution will be basic (pH > 7). (e) (NH4)2S: Comes from weak base NH3 and weak acid H2S. Since H2S is a stronger acid than NH3 is a base, the solution will be basic (pH > 7). (f) NaNO3: Comes from strong base NaOH and strong acid HNO3. The solution will be neutral (pH = 7). (g) Na2HPO4: Comes from strong base NaOH and weak acid H3PO4. This will lead to a basic solution (pH > 7). (h) LiBr: Comes from strong base LiOH and strong acid HBr. The solution will be neutral (pH = 7). (i) FeCl3: Comes from the hydrolysis of Fe3+ ions leading to an acidic solution (pH < 7).

Step 3: Identify Highest and Lowest pH

From the previous step, identify which solutions are acidic, neutral, or basic. The most basic solution (highest pH among basic solutions) is Na2CO3, as CO3^2- is a strong base compared to HPO4^2-. The most acidic solution (lowest pH among acidic solutions) is NaHSO4, as HSO4^- is a part of the strong acid H2SO4.

Key Concepts

Hydrolysis

When salts dissolve in water, they may undergo a process called hydrolysis. This is where either the cation or anion, or both, react with water. This reaction can produce an acidic or basic solution depending on the nature of the parent acid and base of the salt. If a salt's cation comes from a weak base, it can donate an H+ ion to water, creating an acidic solution. Conversely, if a salt's anion comes from a weak acid, it can accept an H+ ion, forming a basic solution.

In our exercise, for example, consider \(\mathrm{NaHSO}_4\). Since \(\mathrm{HSO}_4^-\) is from the strong acid \(\mathrm{H}_2\mathrm{SO}_4\), it tends to release \(\mathrm{H}^+\) ions, thus forming an acidic solution. Understanding hydrolysis helps us grasp why certain salts affect the pH of their solutions.

Acid-Base Chemistry

The foundation of salt solution pH prediction lies in acid-base chemistry. Salts are made from the neutralization of acids and bases, but not all acids and bases are equally strong. To predict pH, identify whether the parent components of the salt are strong or weak.

• If both the acid and base are strong, the resulting solution is neutral (pH = 7).
• If the acid is strong and the base is weak, the solution becomes acidic (pH < 7).
• If the acid is weak and the base is strong, the solution is basic (pH > 7).

For example, \(\mathrm{NaNO}_3\), derived from strong \(\mathrm{HNO}_3\) and strong \(\mathrm{NaOH}\), results in a neutral solution. Meanwhile, \(\mathrm{NH}_4\mathrm{Br}\), which comes from weak \(\mathrm{NH}_3\) and strong \(\mathrm{HBr}\), results in an acidic solution.

pH Prediction

Predicting the pH of a salt solution involves understanding the strengths of the parent acid and base. Let's briefly break down how each salt analyzed in the solution impacts pH. Salts like \(\mathrm{KClO}_4\) and \(\mathrm{LiBr}\) are from strong acids and bases, meaning their solutions will have a neutral pH. Meanwhile, salts like \(\mathrm{FeCl}_3\) become acidic because the metal ion \(\mathrm{Fe}^{3+}\) interacts with water, increasing the solution’s acidity.

To find the highest or lowest pH, compare how basic or acidic different salts are. \(\mathrm{Na}_2\mathrm{CO}_3\) has the highest pH because the \(\mathrm{CO}_3^{2-}\) ion is a strong base. Conversely, \(\mathrm{NaHSO}_4\), closely related to a strong acid, results in the lowest pH in this list.