Question

in 20.0 mL of 1:1 buffer + 5.0 mL HCl

Major species before reaction begins. Include spectator ions.

Major species after reaction occurs. Include spectator ions

Answer 1

Ans. It depends on the type of buffer taken-

I. For a weak acid-conjugate base buffer-

            AH + H₂O <----------> A^- + H₃O⁺

Le Chatelier’s principle states “if a dynamic equilibrium is disturbed by changing the conditions (Concentration, Volume, Pressure, temperature, etc.), the position of equilibrium shifts to counteract the change to reestablish an equilibrium”.

Addition of string acid, HCl shifts the equilibrium to the left. So, the concertation of A^- decreases whereas that of AH increases.

Therefore,

            The major species after reaction = AH

# The major species before reaction: It would depend on the buffer, pKa of weak acid and the pH of the buffer.

# Using Henderson-Hasselbalch equation for weak acid, pH = pKa + log [A^-] / [AH]

Where,            AH = weak acid         ; A^- = Conjugate base of the weak acid.

When, pH = pKa       -          both [A^-] and [AH] are in equal concertation.

            pH > pKa        -          [A^-] > [AH], so [A^-] would be the major species.

            pH < Pka        -          [AH] > [A^-], so [AH] would be the major species.

Note: Kindly correlate the same with your buffer.

# Spectator ion: Remains as the same

2. For a weak base-conjugate acid buffer-

            B + H₂O <---------->BH⁺ + OH^-

Addition of string acid, HCl shifts the equilibrium to the right because H⁺ from acid reacts with OH^- in buffer to form neutral H₂O; that leads BH to react with H2O to equilibrate OH^-. So, the concertation of B decreases whereas that of BH⁺ increases.

Therefore,

            The major species after reaction = BH⁺

# The major species before reaction: It would depend on the buffer, pKa of weak base and the pH of the buffer.

# Using Henderson-Hasselbalch equation for weak acid, pOH = pKb + log ([BH⁺] / [B]); Where                    B = Base , BH⁺ = Conjugate acid

When, pOH = pKb    -          both [B] and [BH⁺] are in equal concertation.

            pOH > pKa    -          [BH⁺] > [B], so [BH⁺] would be the major species.

            pOH < Pka     -          [B] > [BH⁺], so [B] would be the major species.

# Spectator ion: Remains as the same

Note: Kindly correlate the same with your buffer.

Note: pOH + pH = 14.00