   Chapter 14, Problem 181CP

Chapter
Section
Textbook Problem

# Consider the species PO43−, HPO42−, and H2PO4−. Each ion can act as a base in water. Determine the Kb value for each of these species. Which species is the strongest base?

Interpretation Introduction

Interpretation: The Kb value of given species, strongest base among given species is to be stated.

Concept introduction: The formula of Kb is,

Ka×Kb=Kw

Strong bases have high values of Kb .

To determine: The Kb value of given species and the strongest base among given species.

Explanation

Explanation

The value of Kb for H2PO4 is 1.3×10-12_ .

The value of Kb for HPO42 is 1.61×10-7_ .

The value of Kb for PO43 is 0.0208_ .

The given acid H3PO4 is a triprotic acid furnishing 3 protons.

H3PO4H2PO4+H+Ka1=7.5×103H2PO4HPO42+H+Ka2=6.2×108HPO42PO43+H+Ka3=4.8×1013

The anions H2PO42,HPO42 and PO43 acts as bases in aqueous solution and dissociates as,

H2PO42+H2OH3PO4+OH

The formula to calculate the ionic product of water is,

Ka×Kb=Kw

Where,

• Ka is the acid dissociation equilibrium constant.
• Kb is the base dissociation equilibrium constant.
• Kw is the water ionization product.

Substitute the values of Ka and Kw in the above formula.

7.5×103×Kb=1.0×1014Kb=1

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