Problem 55 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following statements about \(\mathrm{pH}\) and \(\mathrm{H}^{+}\) ion concentration is incorrect? (a) addition of one drop of concentrated \(\mathrm{HCl}\) in \(\mathrm{NH}_{4} \mathrm{OH}\) solution decreases \(\mathrm{pH}\) of the solution (b) a solution of the mixture of one equivalent of each of \(\mathrm{CH}_{3} \mathrm{COOH}\) and \(\mathrm{NaOH}\) has a \(\mathrm{pH}\) of 7 (c) \(\mathrm{pH}\) of pure neutral water is not zero (d) a cold and concentrated \(\mathrm{H}_{2} \mathrm{SO}_{4}\) has lower \(\mathrm{H}^{+}\)ion concentration than a dilute solution of \(\mathrm{H}_{2} \mathrm{SO}_{4}\)

Step-by-Step Solution

Answer

Statement (b) is incorrect because the solution is basic, not neutral.

Step 1: Examine Statement A

Statement (a) states that the addition of one drop of concentrated HCl to an NH4OH solution decreases the pH of the solution. Since HCl is a strong acid, adding it to a basic solution like NH4OH, which is a weak base, will cause the pH to decrease. Therefore, statement (a) is correct.

Step 2: Examine Statement B

Statement (b) claims that a solution made from one equivalent each of CH3COOH and NaOH has a pH of 7. One equivalent of NaOH neutralizes one equivalent of CH3COOH, producing a solution of sodium acetate (CH3COONa) and water. Since sodium acetate is a salt of a weak acid and strong base, the resulting solution is slightly basic, meaning the pH should be higher than 7. Therefore, statement (b) is incorrect.

Step 3: Examine Statement C

Statement (c) notes that the pH of pure neutral water is not zero. This is correct because the pH of pure water at 25°C is 7, which is considered neutral.

Step 4: Examine Statement D

Statement (d) says that cold, concentrated H2SO4 has a lower H+ ion concentration than a dilute solution of H2SO4. This is incorrect because concentrated sulfuric acid has a higher concentration of H+ ions compared to its dilute counterpart due to its higher molarity.

Key Concepts

H+ Ion Concentration

The concentration of hydrogen ions (^+]) in a solution is a key factor in determining its H]. The ^+] and H] are inversely related, meaning a high ^+] corresponds to a low H], indicating an acidic environment.
Conversely, a low ^+] equates to a high H], characteristic of a basic solution.
In general:

• Acids increase ^+] by donating ^+] ions.
• Bases reduce ^+] by accepting ^+] ions.

Understanding this relationship is crucial in many chemical applications, such as predicting the behavior of a solution in acid-base reactions or calculating the changes in H] during titrations. For instance, adding sulfuric acid (H2SO4) to water will increase the ^+] concentration, lowering the H] of the solution.

Acid-Base Reactions

Acid-base reactions are fundamental in chemistry, involving the transfer of protons (^+] ions) between substances. An acid-base reaction typically results in the formation of water and a salt.
In such reactions:

• An acid donates an ^+] ion.
• A base accepts the ^+] ion.

These reactions play a vital role not only in laboratory chemistry but also in biological and industrial processes. For example, when hydrochloric acid (HCl) reacts with ammonia (NH3), water and ammonium chloride are produced.
This concept helps in understanding the outcomes from Step 1 of the solution, where adding a strong acid (HCl) to a weak base (NH4OH) decreases the overall H] of the solution.

Neutralization

Neutralization is a specific type of acid-base reaction where an acid and a base react to form water and a salt.
The reaction results in the cancellation of each other's properties, leading ideally to a H] of 7, which represents a neutral solution. However, not all neutralization reactions result in a pure pH of 7.
This occurs because:

• Strong acids strong bases yield H] of 7.
• Weak acids strong bases result in a basic solution (H] > 7).

Sodium acetate (CH3COONa) is produced in the reaction of a weak acid (acetic acid) and a strong base (sodium hydroxide). The presence of sodium acetate can lead to a basic solution, explaining why the statement about achieving a H] of 7 in this context is incorrect.

Sodium Acetate

Sodium acetate (CH3COONa) is a salt typically formed from the neutralization of acetic acid (a weak acid) by sodium hydroxide (a strong base).
In solution, it partially dissociates to form acetate ions (CH3COO-) and sodium ions (Na+).

• Acetate ion can accept a proton (^+]), making solutions of sodium acetate slightly basic.
• This is why solutions of sodium acetate have a H] greater than 7, rather than neutral.

Understanding the role of sodium acetate in solutions is important, as it explains the basic nature resulting from its dissociation due to the predominance of a strong base during its formation. This plays a crucial role in buffer solutions and other chemical applications, influencing the H] of environments.