What is the equilibrium pH of the solution? How much of sodium hydroxide (NaOH)

Question

What is the equilibrium pH of the solution?

How much of sodium hydroxide (NaOH) should be added to the solution to adjust the solution pH as 7.0?

How much of sodium hydroxide (NaOH) should be added to the solution to adjust the solution pH as 7.0?

Lactic acid (Molecular weight = 90) is a monoprotic acid and can be dissociated in water as follows. Lactic acid Lactate + H+ (pKa = 3.9) 113 g of sodium lactate is dissolved into 1 L deionized water to make a solution. Thermodynamic Constants for Species of Importance in Water Chemistry Values of Constants Metric Equivalents of Standard Units Thermodynamic Constants for Species of Importance in Water Chemistry

Explanation / Answer

AS sodium lactate is a salt of weak acid and stron base , it undegoes salt hydrolyis.

Use the concept of salt hydrolysis to solve this problem

given 113 g of sodium lactate

molar mass of sodium lactate = 112

so moles of sodium lactate = mass /molar mass

moles of lactate = 113/112

moles of lactate =1.0089

volume = 1L

we know that

conc = moles /volume

conc = 1.0089 /1

conc = 1.0089

the reaction is given by

lactate + H+ ----> lactic acid

Kb = [ products / reactants ]

Kb = x2 / ( 1.0089-x)

given pKa = 3.9

-log Ka = 3.9

Ka = 1.2589 x 10-4

we know that

Kb = Kw /Ka

Kb = 10-14 / 1.2589 x 10-4

Kb = 7.943 x 10-11

so we know that

Kb = x2 / ( 1.0089-x)

solving for x we get

x = 0.00000895188

[OH- = x = 0.00000895188

pOH = -log [OH-]

pOH = -log [ 0.00000895188]

pOH = 5.048

pH = 14 - 5.048

pH = 8.95

so the pH is 8.95

You cannot bring the pH back from 8.95 to 7 by adding NaOH . As NaOH increases the pH . You need to add an acid to bring the pH down to 7

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