A buffer solution is composed of 1.360 g of KH₂PO₄ and 5.677 g of Na₂HPO₄.

1. (a) What is the pH of the buffer solution?
2. (b) What mass of KH₂PO₄ must be added to decrease the buffer solution pH by 0.50 unit from the value calculated in part (a)?

Interpretation:

For the given buffer solution H2PO4−/HPO42− of calculate (a) the value of when 1.360 g of KH2PO4 and 5.677 g of Na2HPO4 are mixed. (b) the mass of KH2PO4 must be added to decrease the  pH value of the buffer 0.50 unit from the value of pH in part (a).

Concept introduction:

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

pH=pKa+log[conjugate base][acid] (1)

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

The calculation of pH is done by using Henderson-Hasselbalch equation. The net ionic equilibrium between H2PO4− and its conjugate base, HPO42− is written as,

H2PO4−(aq)+H2O(l)⇌H3O+(aq) + HPO42−(aq)  (acid)                                             (conjugate base)

Given:

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

The value of Ka for H2PO4− is 6.2×10−8.

Negative logarithm of the Ka value gives the pKa value of the acid.

pKa=−log(Ka)=−log(6.2×10−8)=7.20

Therefore, pKa value for the H2PO4− is 7.20.

The amount of KH2PO4 dissolved is 1.360 g.

The amount of Na2HPO4 dissolved is 5.677 g.

The molecular mass of KH2PO4 is 136.0 g⋅mol−1.

The molecular mass of Na2HPO4 is 141.96 g⋅mol−1.

The concentration of KH2PO4 and Na2HPO4 is calculated by using the following expression.

Molarity=weight(molecular mass)(1 L volume of solvent)(mol⋅L−1) (2)

Substitute, 1.360 g for weight, 136.0 g⋅mol−1 for molecular mass.

Molarity=1.360 g(136.0 g⋅mol−1)(1 L volume of solvent)(mol⋅L−1)=0

Interpretation:

For the given buffer solution H2PO4−/HPO42− of calculate (a) the value of when 1.360 g of KH2PO4 and 5.677 g of Na2HPO4 are mixed. (b) the mass of KH2PO4 must be added to decrease the  pH value of the buffer 0.50 unit from the value of pH in part (a).

Concept introduction:

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

pH=pKa+log[conjugate base][acid] (1)

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