Question

the structural fomula and fraction present calculated to three significant digits.

a.citric acid at ph=2,6,10

b.threonine at ph=2,6,10

c.Glutamic acid at ph=2,6,10

Answer 1

Solution a:

The structure of citric acid at pH = 2, 6 is

Citric acid is tri basic acid with pKa₁, pKa₂, pKa₃ are 3.13, 4.76, 6.40 respectively. Since Ka₃ = 3.9 x 10 ^{– 7} , the dissociation is too less than [H⁺ ], thus we ignore the third dissociation of this acid towards pH contribution.

We know, pH = - log [H⁺ ] thus [H⁺ ] = 10 ^{– pH} .

Consider the following terminology from the dissociation of this equation.

The fraction of [CA] = [H⁺ ]² /{[H⁺ ]²+Ka₁[H⁺ ]+Ka₁xKa₂}

The fraction of [CA^{- 1} ] = Ka₁ [H⁺ ] /{[H⁺ ]²+[H⁺ ]+Ka₁xKa₂}

The fraction of [CA^{- 2} ] = Ka₁ xKa₂/{[H⁺ ]²+Ka₁[H⁺ ]+Ka₁xKa₂}

At pH = 2, [H⁺ ] = 10 ^{– 2} M

The fraction of [CA] = 0.931

The fraction of [CA^{- 1} ] = 7.32 x 10 ^{– 4}

The fraction of [CA^{- 2} ] = 1.19 x 10 ^{– 4}

At pH = 6, [H⁺ ] = 10 ^{– 6} M

The fraction of [CA] = 7.38 x 10 ^{– 5}

The fraction of [CA^{- 1} ] = 7.31 x 10 ^{– 4}

The fraction of [CA^{- 2} ] = 12.8

At pH = 10,

The structure is a follows and it is a buffer and fraction depends on the concentration of citric salt taken in the solution.

Solution b:

the structure of threonine at pH = 2 is as follows and the fraction of threonine = [H⁺ ]² /Ka = 0.0218 (pKa = 2.34)

At pH = 6, the threonine exits as follows and fraction is equal to concentration taken.

At pH = 10, Structure as follows and it is a buffer and fraction depends on the concentration of citric salt taken in the solution.

Solution c:

the structure of Glutamic acid at pH = 2 is as follows and the fraction of threonine = [H⁺ ]² /Ka = 1.77 (pKa = 4.25)

At pH = 6, 10 the acid exist as follows as its iso-electric point is at 4.25