Question

The three $p{K_a}$ values of aspartic acid are 1.89, 3.65 and 9.60. The $pH$ of the amino acid is?
A. 2.77
B. 6.62
C. 5.74
D. 7.0

Answer 1

To find the solution you need to know the formula for finding the pH from $p{K_a}$ values. The $pH$ is obtained by taking the average of the two $p{K_a}$ values. But you need to find out which two values should be used to take the average.

Step by step answer:
Let’s discuss the answer step by step. Now, we need to break the question into parts. First let’s know something about aspartic acid. Aspartic acid is an amino acid that is used in the biosynthesis of proteins. Like all other amino acids it contains amino groups and a carboxylic acid. Aspartic acid is also used by athletes to increase mineral levels and enhance their performance.

Now, what is $p{K_a}$ ? The $p{K_a}$ value is used to indicate the strength of an acid. It is the negative base 10 logarithm of the acid dissociation constant ( ${K_a}$ ) or acidity constant of a solution. It is the equilibrium constant for a chemical reaction. Lower the $p{K_a}$ value, stronger is the acid. Also, lower value indicates that the acid fully dissociates in water. The $p{K_a}$ representation is used because it describes acid dissociation using small decimal numbers.

Now, here we have to find the $pH$ value of the amino acid. To find the pH value we need to find the average of two of the $p{K_a}$ values. But here we have three values. But to find the $pH$ we take two of the values which are more acidic than the others. We know that lower the $p{K_a}$ value is higher is its acidity. Therefore, the smallest values are 1.89 and 3.65. Therefore, we will use these two values in the equation.
The equation is given below:
$pH = \dfrac{{p{K_a}_{_1} + p{K_a}_{_2}}}{2}$
Now, substituting the values of $p{K_{{a_1}}}$ and $p{K_{{a_2}}}$ we get
$pH = \dfrac{{1.89 + 3.65}}{2} = 2.77$

Therefore, here we get our answer as 2.77. So, the answer is option A.

Note: The important thing to note here is the values used in the solution and the formula that will be used. Not knowing any one of the things may lead to wrong answers.