Problem 75 - A Complete Resource Book in Chemistry for JEE Main

Question

Which has highest \(\mathrm{pH} ?\) (a) \(\mathrm{CH}_{3} \mathrm{COOK}\) (b) \(\mathrm{Na}_{2} \mathrm{CO}_{3}\) (c) \(\mathrm{NH}_{4} \mathrm{Cl}\) (d) \(\mathrm{NaNO}_{3}\)

Step-by-Step Solution

Answer

\( \mathrm{Na}_2 \mathrm{CO}_3 \) has the highest pH.

Step 1: Understand the Compounds

Let's look at the chemical nature of each of the compounds given and determine their acid/base properties. (a) \( \mathrm{CH}_3 \mathrm{COOK} \) is a salt derived from a weak acid (acetic acid, \( \mathrm{CH}_3 \mathrm{COOH} \)) and a strong base (\( \mathrm{KOH} \)).(b) \( \mathrm{Na}_2 \mathrm{CO}_3 \) is a salt derived from a weak acid (carbonic acid, \( \mathrm{H}_2 \mathrm{CO}_3 \)) and a strong base (\( \mathrm{NaOH} \)).(c) \( \mathrm{NH}_4 \mathrm{Cl} \) is a salt from a strong acid (hydrochloric acid, \( \mathrm{HCl} \)) and a weak base (ammonia, \( \mathrm{NH}_3 \)).(d) \( \mathrm{NaNO}_3 \) is a salt from a strong acid (nitric acid, \( \mathrm{HNO}_3 \)) and a strong base (\( \mathrm{NaOH} \)).

Step 2: Determine the pH Characteristics

Given the acid/base nature of each component in the salts:(a) \( \mathrm{CH}_3 \mathrm{COOK} \) forms a basic solution because it comes from a weak acid and a strong base, so the conjugate base \( \mathrm{CH}_3 \mathrm{COO}^- \) will remain.(b) \( \mathrm{Na}_2 \mathrm{CO}_3 \) also forms a basic solution because it comes from a weak acid and a strong base, with the bicarbonate \( \mathrm{CO}_3^{2-} \) acting as a base.(c) \( \mathrm{NH}_4 \mathrm{Cl} \) forms an acidic solution as it comes from a strong acid and a weak base, with \( \mathrm{NH}_4^+ \) acting as an acid.(d) \( \mathrm{NaNO}_3 \) is neutral because it is derived from both strong acid and strong base.

Step 3: Compare Basicity of CH3COOK and Na2CO3

Between the basic salts \( \mathrm{CH}_3 \mathrm{COOK} \) and \( \mathrm{Na}_2 \mathrm{CO}_3 \), \( \mathrm{Na}_2 \mathrm{CO}_3 \) is more basic because the carbonate ion \( \mathrm{CO}_3^{2-} \) is a stronger base and can accept more protons compared to the acetate ion \( \mathrm{CH}_3 \mathrm{COO}^- \). Thus, \( \mathrm{Na}_2 \mathrm{CO}_3 \) has a higher pH value.

Key Concepts

Acid-Base Properties

To understand the pH of salts, it's essential first to explore their acid-base properties. Salts are formed from the reaction between an acid and a base. When this happens, the nature of the resulting salt can impact the pH of a solution it is dissolved in. The salt's pH is determined by the strengths of the acid and base from which it is derived.
For instance:

• If a salt is formed from a strong acid and a strong base, the resulting solution is usually neutral, meaning it has a pH around 7.
• A salt resulting from a strong acid and a weak base will lead to an acidic solution, lowering the pH below 7.
• Conversely, if a salt is derived from a weak acid and a strong base, the solution will be basic, raising the pH above 7.

Knowing these properties helps predict the effect a salt will have when dissolved in water, affecting its potential applications in different chemical settings.

Weak Acid Strong Base Salts

When a salt originates from a weak acid and a strong base, it typically creates a basic solution. This happens because the strong base can neutralize the weak acid, leaving behind the conjugate base of the weak acid to dominate the solution's properties.
Examples include:

• Potassium acetate (\( \mathrm{CH}_3 \mathrm{COOK} \) - from acetic acid and potassium hydroxide).
• Sodium carbonate (\( \mathrm{Na}_2\mathrm{CO}_3 \) - from carbonic acid and sodium hydroxide).

In these cases, ions like the acetate ion (\( \mathrm{CH}_3 \mathrm{COO}^- \)) or carbonate ion (\( \mathrm{CO}_3^{2-} \)) remain in the solution. These ions are capable of accepting protons, which increases hydroxide ion concentration (\( \mathrm{OH}^- \)), resulting in a basic pH.

Strong Acid Weak Base Salts

Salts resulting from a strong acid and a weak base tend to create acidic solutions when dissolved in water. This occurs because the strong acid is fully dissociated in water, while the weak base is not wholly neutralized, leaving its conjugate acid in the solution.

• Example: Ammonium chloride (\( \mathrm{NH}_4 \mathrm{Cl} \)) is derived from hydrochloric acid and ammonia.

In such cases, ions like the ammonium ion (\( \mathrm{NH}_4^+ \)) can donate protons, increasing the hydrogen ion (\( \mathrm{H}^+ \)) concentration, thereby lowering the pH. Hence, these solutions are characteristically acidic in nature.

Neutral Salts

Neutral salts emerge from the reaction of a strong acid and a strong base. Since both ions in these salts are fully dissociated and neither has a residue of acidic nor basic activity, the resulting solution remains neutral with a pH close to 7.

• Example: Sodium nitrate (\( \mathrm{NaNO}_3 \)) from nitric acid and sodium hydroxide.

In neutral solutions, the conjugate base of the strong acid and the conjugate acid of the strong base do not affect the pH significantly. Therefore, these salts are often used in settings where a neutral pH is desired, due to their ability to maintain this balance.