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Amino Acids

The structure...

Question

The structure of a basic amino acid, lysine, is given. The $p K_{a_{1}}, p K_{a_{2}}$ and $p K_{a_{3}}$ of $(A)$ are: $2.18, 8.95$ and $10.53$ .
What is the $p H$ at isoelectric points $(p I)$ ?

A

\frac{p K_{a_{1}} + p K_{a_{2}}}{2}

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B

\frac{p K_{a_{1}} + p K_{a_{3}}}{2}

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C

\frac{p K_{a_{2}} + p K_{a_{3}}}{2}

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D

\frac{p K_{a_{1}} + p K_{a_{2}} + p K_{a_{3}}}{3}

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Solution

The correct option is C $\frac{p K_{a_{2}} + p K_{a_{3}}}{2}$
If there are more than one amine group then we will take the average of $p K_{a}$ of amine groups.
$p I$ = $\frac{p K_{a_{2}} + p K_{a_{3}}}{2}$

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Similar questions

Lysine has p K_{a_{1}} = 2.18, p K_{a_{2}} = 8.95, p K_{a_{3}} = 10.53

In which structure lysine will be present at

p H = 9.7

.

⎛ ⎜ ⎜ ⎝ H O O C - C | H^{\oplus} N H_{3} - C H_{2} C O O H ⎞ ⎟ ⎟ ⎠

The structure of aspartic acid is given.The

p K_{a_{1}}, p K_{a_{2}}

p K_{a_{3}}

(A)

respectively are:

1.88, 3.65

9.60

p K_{a_{1}}

corresponds to the ionisation of the

C O O H

⎛ ⎜ ⎜ ⎜ ⎜ ⎝ R \oplus N H_{3} | C H - C O O H ⎞ ⎟ ⎟ ⎟ ⎟ ⎠

p K_{a_{2}}

corresponds to the ionisation of

(\oplus N H_{3})

(ammonium ion). What is the

p H

at isoelectric points

(p I)

?

p K_{a_{1}}, p K_{a_{2}} a n d p K_{a_{3}}

H_{3} P O_{4}

are 2.2, 7.2 and 12.3 respectively. The

p H

0.1 M

N a_{2} H P O_{4}

solution will be:

Q. A one litre solution contains

0.04 M

N a_{2} H P O_{4}

0.02 M

N a H_{2} P O_{4}

. The correct statement(s) is/are:

H_{3} P O_{4} :

p K_{a_{1}} = 2.14; p K_{a_{2}} = 6.84; p K_{a_{3}} = 12.3

l o g 2 = 0.3

Q. Which of the following relation is correct regarding

p H

of salt of amphiprotic ion,

H_{2} P O_{4}^{-}

K_{a_{1}}

is first acid dissociation constant of

H_{3} P O_{4}

K_{a_{2}}

is second acid dissociation constant of

H_{3} P O_{4}

.

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