If a buffer solution is 0.160 M in a weak acid (Ka = 1.0 × 10-5)...

If a buffer solution is 0.160 M in a weak acid (Ka = 1.0 × 10-5) and 0.470 M in its conjugate base, what is the pH?

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Expert Solution

There are two ways to solve this problem.

First method:

Firatwe should calculate pKa value

pKa = -log Ka = -log (1.0 × 10^-5)

pKa = 5

We know pKa value, weak acid concentration (0.160 M) and conjugate base concentration (0.470 M)

According to Henderson-Haselbalch equation: pH = pKa + log [base]/[acid]

Substitute all value in top equation

pH = 5 + log [0.470]/[0.160] = 5 + log [2.9375] = 5+0.467

pH = 5.467

Second method:

pH = -log[H⁺]

we have to calculate H⁺ ion concentration

the equilibrium equation between the weak acid and its conjugate acid is:

AH <--> H⁺+ A^-
we know Ka value (1.0 x 10^-5)
Ka= [H⁺][A^-]/[AH] = 1.0 X 10^-5
we know weak acid concentration ([AH] = 0.160 M) and conjugate base concentration ([A^-]=[0.470 M) substitute this values in top equation

Ka= [H⁺][0.47]/[0.16] = 1.0 X 10^-5
1.0 X 10^-5 = (0.470) [H⁺] / (0.160)
[H⁺] = 3.40 X 10^-6

then pH = -log[H⁺]

pH = - log 3.40 X 10^-6 = 5.46

queen_honey_blossom answered 3 weeks ago

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