The ionisation constant of an acid-base indicator (a weak acid) is 1.0 × 10-6. The ionized formed of the indicator is red and unionized form is blue. The pH change required to alter the colour of indicator from 80 % red is

Question

The ionisation constant of an acid-base indicator (a weak acid) is 1.0 × 10-6. The ionized formed of the indicator is red and unionized form is blue. The pH change required to alter the colour of indicator from 80 % red is (A) 0.80 (B) 1.20 (C) 1.40 (D) 2.00

Tardigrade Admin · Accepted Answer

HIn leftharpoons H ⊕+ operatornameIndΘ pH =p K text Ind + log ( text Ind Θ/[ HIn ]) pH 1=p K text Ind + log (20/80)=p K text Ind =2 log 2 pH 2=p K text Ind + log (80/20)=p K text Ind +2 log 2 Δ( pH )= pH 2= pH 1=4 log 2=4 × 0.3=1.2

Q. The ionisation constant of an acid-base indicator (a weak acid) is $1.0 \times 1 0^{- 6}$ . The ionized formed of the indicator is red and unionized form is blue. The $p H$ change required to alter the colour of indicator from $80%$ red is

$0.80$

$1.20$

$1.40$

$2.00$

$H I n ⇌ H^{\oplus} + Ind^{Θ}$
$p H = p K_{Ind} + lo g \frac{Ind ^{Θ}}{[ H I n ]}$
$p H_{1} = p K_{Ind} + lo g \frac{20}{80} = p K_{Ind} = 2 lo g 2$
$p H_{2} = p K_{Ind} + lo g \frac{80}{20} = p K_{Ind} + 2 lo g 2$
$Δ (p H) = p H_{2} = p H_{1} = 4 lo g 2 = 4 \times 0.3 = 1.2$

Q. The ionisation constant of an acid-base indicator (a weak acid) is 1.0×10−6. The ionized formed of the indicator is red and unionized form is blue. The pH change required to alter the colour of indicator from 80% red is

0.80

1.20

1.40

2.00