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a. Which is a stronger acid, one with a pKa of 5.2 or one
Chapter 2, Problem 4P(choose chapter or problem)
Problem 4P
a. Which is a stronger acid, one with a pKa of 5.2 or one with a pKa of 5.8?
b. Which is a stronger acid, one with an acid dissociation constant of 3.4 X10−3 with an acid dissociation constant of 2.1 X1CT4 ?
Questions & Answers
QUESTION:
Problem 4P
a. Which is a stronger acid, one with a pKa of 5.2 or one with a pKa of 5.8?
b. Which is a stronger acid, one with an acid dissociation constant of 3.4 X10−3 with an acid dissociation constant of 2.1 X1CT4 ?
ANSWER:
Solution 4P :
Step 1 of 3:
Here, we need to find the stronger acid with respect to pKa and dissociation constant. First, let’s see what is strength of acids.
Strength of acids is the ability of an acid to lose protons(H+). A strong acid is one that completely dissociates in a solution, that is it loses its protons completely and a weak acid partially dissociates in solution. The strength of acidity is measured through dissociation constant, represented as Ka.
We calculate the strength of acidity using the formula :
pKa = -log (Ka)
where, ‘p’ stands for ‘potential of’.
When Ka is calculated, usually the results are an extremely large number. Hence, for convenience, chemists simplified it to a smaller number, called pKa. pKa is the negative logarithm of the dissociation constant, Ka.
The smaller the pKa value, stronger is the acid, and the higher the pKa value, weaker is the acid. Inversely, the smaller the Ka value, weaker is the acid, and the higher the Ka value, stronger is the acid.
The strength of acidity with pKa is related as shown in the table below :
Strength of acids |
pKa value |
very strong acids |
< 1 |
moderately strong acids |
1-3 |
weak acids |
3-5 |
very weak acids |
5-15 |
extremely weak acids |
>15 |