   Chapter 15, Problem 102CWP

Chapter
Section
Textbook Problem

# Consider the following acids and bases: HCO2H Ka = 1.8 × 10−4 HOBr Ka = 2.0 × 10−9 (C2H5)2NH Kb = 1.3 × 10−3 HONH2 Kb = 1.1 × 10−8 Choose substances from the following list that would be the best choice to prepare a pH = 9.0 buffer solution.a. HCO2Hb. HOBrc. KHCO2d. HONH3NO3e. (C2H5)2NHf. (C2H5)2NH2Clg. HONH2h. NaOBr

Interpretation Introduction

Interpretation: To prepare a buffer solution of pH-9, the pair of acid base is to be chosen from the given combination.

Concept introduction: The buffer solutions are the solutions which resist the pH of the solution even on the external addition of the acid or the base.

The buffer solution having the pH-7 is known as a neutral buffer solution.

In the acidic buffer solution, the acid with their salt is present while in basic buffer, the base with the respective salt is present.

If the pH of the buffer solution is more than 7, then it is called basic buffer and if less, then it is acidic.

The hydrogen ion concentration of the solution is known as pH of the solution.

It is the negative logarithm of Hydrogen ion concentration.

Explanation

Given

The given compounds with their respective Ka and Kb value are,

HCOOHKa=1.8×104HOBrKa=2.0×109(C2H5)NHKb=1.3×103NH2OHKb=1.1×108

The pH of the buffer solution is 9.0.

The given pH value clearly indicates that the buffer solution is basic buffer.

pKa is the negative logarithm of ka.

For any basic buffer, pH range is always ±1 to pKa.

Therefore,

pH=pKa±1

Substitute the value of pH of 9 in the above equation.

pH=pKa±19=pKa±1pKa=9±1

Therefore, the value of pKa lies in the range 810

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