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Chapter 17, Problem 25PS
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### Chemistry & Chemical Reactivity

10th Edition
John C. Kotz + 3 others
ISBN: 9781337399074

#### Solutions

Chapter
Section
BuyFindarrow_forward

### Chemistry & Chemical Reactivity

10th Edition
John C. Kotz + 3 others
ISBN: 9781337399074
Textbook Problem

# A buffer solution is prepared by adding 0.125 mol of ammonium chloride to 5.00 × 102 mL of 0.500 M solution of ammonia. (a) What is the pH of the buffer? (b) If 0.0100 mol of HCl gas is bubbled into 5.00 × 102 mL of the buffer, what is the new pH of the solution?

(a)

Interpretation Introduction

Interpretation:

For the buffer solution which is prepared by adding 0.125 mol of ammonium chloride in 5.0×102mL of 0.500 M NH3 . pH of the buffer solution has to be calculated.

Concept introduction:

The Henderson-Hasselbalch equation relates pOH of a buffer with pKb of base, concentration of conjugate acid and concentration of base. The expression is written as,

pOH=pKb+log[conjugateacid][base] (1)

This equation shows that pOH of buffer solution is controlled by two major factors. First, is strength of the base which can be expressed on terms of pKb and second, the relative concentration of base and its conjugate acid at equilibrium.

The pH value is calculated by expression, pH + pOH = 14.

Explanation

The calculation of pH is done by using Henderson-Hasselbalch equation.

Given:

Refer to table 16.2 in the textbook for the value of Kb.

The value of Kb for NH3 is 1.8Ã—10âˆ’5.

The pKb value is calculated as follows;

pKb=âˆ’log(Kb)

Substitute, 1.8Ã—10âˆ’5 for Kb.

pKb=âˆ’log(1.8Ã—10âˆ’5)=4.74

Therefore, pKb value of NH3 is 4.74.

The initial concentration of NH3 is 0.500Â molâ‹…Lâˆ’1.

The initial moles of NH4Cl is 0.125Â mol.

The volume of the buffer solution is 5Ã—102mL.

Unit conversion of 5Ã—102mL into L.

(5Ã—102Â mL)(1Â L1000Â mL)=0.500Â L

Therefore, volume of the solution is 0.500Â L.

The concentration of NH4Cl is calculated as follows;

Molarity=NumberÂ ofÂ molesvolumeÂ ofÂ solvent (2)

Substitute 0.125Â mol for number of moles and 0.500Â L for volume of solvent in equation (2).

Molarity=0

(b)

Interpretation Introduction

Interpretation:

For the buffer solution which is prepared by adding 0.125 mol of ammonium chloride in 5.0×102mL of 0.500 M NH3. pH of the solution when 0.0100 mol of HCl is added to 5.0×102 mL of buffer solution has to be calculated.

Concept introduction:

The Henderson-Hasselbalch equation relates pOH of a buffer with pKb of base, concentration of conjugate acid and concentration of base. The expression is written as,

pOH=pKb+log[conjugateacid][base] (1)

This equation shows that pOH of buffer solution is controlled by two major factors. First, is strength of the base which can be expressed on terms of pKb and second, the relative concentration of base and its conjugate acid at equilibrium.

The pH value is calculated by expression, pH + pOH = 14.

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