Question

The pH of 1 N aqueous solutions of HCl, CH$_3$COOH and HCOOH follows the order:

• A. HCl $>$ HCOOH $>$ CH$_3$COOH
• B. HCl $=$ HCOOH $>$ CH$_3$COOH
• C. CH$_3$COOH $>$ HCOOH $>$ HCl
• D. CH$_3$COOH $=$ HCOOH $>$ HCl

Hint:

The stronger the acid, the lower its pH value. Therefore, the pH order is the reverse of the acid strength order.

Answer 1

The Correct Option is C

Step 1: {Understanding the Strength of Acids} 
Stronger the acid, lower is the pH, and weaker the acid, higher the pH. 
Step 2: {Order of Acid Strength} 
The strength of the given acids is in the order: \[ {HCl}>{HCOOH}>{CH}_3{COOH} \] 
Step 3: {Order of pH} 
Hence, their pH is in the order: \[ {CH}_3{COOH}>{HCOOH}>{HCl} \] Thus, the correct answer is (C). 
 

Answer 2

Step 1: Understand the nature of the acids
We are comparing the pH values of 1 N (normal) aqueous solutions of three acids:
- HCl: Strong acid
- HCOOH (formic acid): Weak acid
- CH₃COOH (acetic acid): Weak acid

Step 2: Analyze strength and dissociation
HCl: Completely dissociates in water, releasing a high concentration of H⁺ ions.
⇒ Produces low pH (highly acidic)

HCOOH: Weak acid, partially dissociates in water.
Ka of formic acid ≈ \(1.8 \times 10^{-4}\)

CH₃COOH: Also a weak acid, but weaker than formic acid.
Ka of acetic acid ≈ \(1.8 \times 10^{-5}\)
Since it dissociates even less than formic acid, it produces fewer H⁺ ions in solution.

Step 3: Order of pH
- HCl: Strong acid → most H⁺ → lowest pH
- HCOOH: more dissociation than CH₃COOH → lower pH than acetic acid
- CH₃COOH: least dissociation → least H⁺ → highest pH

Step 4: Final order
pH: CH₃COOH > HCOOH > HCl

Final Answer: CH₃COOH > HCOOH > HCl