   Chapter 15, Problem 50E

Chapter
Section
Textbook Problem

# Calculate the pH of a solution that is 0.20 M HOCl and 0.90 M KOCl. In order for this buffer to have pH = pKa, would you add HCl or NaOH? What quantity (moles) of which reagent would you add to 1.0 L of the original buffer so that the resulting solution has pH = pKa?

Interpretation Introduction

Interpretation:

The concentration of HOCl and KOCl is given. The pH value of this solution is to be calculated. The compound to be added, between HCl and NaOH , in order to have pH=pKa is to be identified. The quantity (moles) of reagent that would be added to 1.0L of original buffer so that the resulting solution has pH=pKa is to be calculated.

Concept introduction:

A solution that contains mixture of a weak acid and its conjugate base is known as buffer solution. The pH value is the measure of H+ ions. The best buffer is prepared when the concentration of weak acid is equal to its conjugate base, that is [Acid]=[Base] .

To determine: The pH of solution of HOCl and KOCl ; if HCl or NaOH is added in order to have pH=pKa ; the quantity (moles) of reagent that would be added to 1.0L of original buffer.

Explanation

Explanation

To find the pH of solution of HOCl and KOCl

The concentration of HOCl is 0.20M .

The concentration of KOCl is 0.90M .

The value of Ka for HOCl is 3.5×108 .

Formula

The pH is calculated using the Henderson-Hasselbach equation,

pH=pKa+log[OCl][HOCl]

Where,

• pH is the measure of H+ ions.
• pKa is the measure of acidic strength.
• [OCl] is concentration of (OCl) .
• [HOCl] is concentration of (HOCl) .

The formula of pKa is,

pKa=logKa

Where,

• Ka is acid equilibrium constant.

Substitute the value of pKa in the above equation.

pH=logKa+log[OCl][HOCl]

Substitute all the given values in Henderson-Hasselbach equation.

pH=logKa+log[OCl][HOCl]=log(3.5×108)+log[0.90][0.20]=8.11_

To find the acid that is needed to be added

In order to have pH=pKa , the concentration of weak acid is equal to its conjugate base, that is [Acid]=[Base] .

But the concentration of acid is less than the concentration of base. Hence, in order to have [Acid]=[Base] , the concentration of acid should be increased. This can only happen when more acid is added to the solution of HOCl and KOCl . Therefore, HCl must be added to the buffer solution.

To show that the resulting solution has pH=pKa

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