Phosphoric acid ionizes in three steps with their ionization constant values Ka1, Ka2 and Ka3, respectively, while K is the overall ionization constant. Which of the following statements are true? A. log K = log Ka1 + log Ka2 + log Ka3 B. H3PO4 is a stronger acid than H2PO4- and HPO42-. C. Ka1 > Ka2 > Ka3 D. Ka1 = (Ka3 + Ka2) / 2 Choose the correct answer from the options given below: (1) A and B only (2) A and C only (3) B, C and D only (4) A, B and C only

The correct answer is Option 4. Full mathematical solution steps and hints are accessible in the interactive StemCET workspace.

Chemistry Q59 – NEET-UG 2025

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Home Chemistry EquilibriumQuestion #433

Phosphoric acid ionizes in three steps with their ionization constant values

K_{a_{1}}

K_{a_{2}}

and

K_{a_{3}}

, respectively, while

K

is the overall ionization constant. Which of the following statements are true? A.

\log K = \log K_{a_{1}} + \log K_{a_{2}} + \log K_{a_{3}}

H_{3} {PO}_{4}

is a stronger acid than

H_{2} {PO}_{4}^{-}

and

{HPO}_{4}^{2 -}

. C.

K_{a_{1}} > K_{a_{2}} > K_{a_{3}}

K_{a_{1}} = \frac{K_{a_{3}} + K_{a_{2}}}{2}

Choose the correct answer from the options given below:

Clue 1 of 3: Polyprotic Acids

Understand the concept of successive ionization constants for polyprotic acids and how they relate to overall ionization.

🧠 Full Solution Path

Step 1: Relate Overall and Stepwise Ionization Constants✦ Active

For a polyprotic acid, the overall ionization constant $K$ is the product of the individual stepwise ionization constants $K_{a_{1}}$ , $K_{a_{2}}$ , and $K_{a_{3}}$ .

K = K_{a_{1}} \times K_{a_{2}} \times K_{a_{3}}

Taking the logarithm of both sides, we get:

\log K = \log (K_{a_{1}} \times K_{a_{2}} \times K_{a_{3}}) = \log K_{a_{1}} + \log K_{a_{2}} + \log K_{a_{3}}

Therefore, statement A is true.

💡 Teacher's Secret Hint

Remember that the overall equilibrium constant for a series of reactions is the product of the individual equilibrium constants.

Step 2: Evaluate Acid Strength and Relative

K_{a}

Values○ Expand

For a polyprotic acid like phosphoric acid ( $H_{3} {PO}_{4}$ ), the acidity decreases with each successive deprotonation. This means the original acid is stronger than its conjugate base formed after the first deprotonation, and so on.

Thus, $H_{3} {PO}_{4}$ is a stronger acid than $H_{2} {PO}_{4}^{-}$ , which is stronger than ${HPO}_{4}^{2 -}$ . Therefore, statement B is true.

Correspondingly, the ionization constants also decrease with each step: $K_{a_{1}} > K_{a_{2}} > K_{a_{3}}$ . Therefore, statement C is true.

💡 Teacher's Secret Hint

The removal of a proton from an already negatively charged species is more difficult due to increased electrostatic attraction, leading to lower acidity.

Step 3: Evaluate Statement D and Identify Correct Option○ Expand

Statement D suggests $K_{a_{1}} = \frac{K_{a_{3}} + K_{a_{2}}}{2}$ . This is an arithmetic mean relationship that does not hold true for successive ionization constants of polyprotic acids. $K_{a_{1}}$ is typically orders of magnitude larger than $K_{a_{2}}$ and $K_{a_{3}}$ . Therefore, statement D is false.

Since statements A, B, and C are true, the correct option is the one that includes A, B, and C.

💡 Teacher's Secret Hint

Always verify mathematical relationships against fundamental chemical principles.

✦ STEM Console utilizes AI models to generate step-by-step explanations and math clues. AI can make mistakes.

Chemistry Question 59 – NEET-UG 2025

Clue 1 of 3: Polyprotic Acids

🧠 Full Solution Path