Question

Among the following acids which has the lowest pKa value? (a) \(\mathrm{CH}_{3} \mathrm{COOH}\) (b) \(\mathrm{HCOOH}\) (c) \(\left(\mathrm{CH}_{3}\right)_{2} \mathrm{CH}-\mathrm{COOH}\) (d) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{COH}\)

Short answer

Formic acid (\(\mathrm{HCOOH}\)) has the lowest pKa value.

Step-by-step solution

Understanding pKa

The pKa value of an acid reflects its acid strength. A lower pKa indicates a stronger acid because it means the compound ionizes more completely in solution compared to acids with higher pKa values.

Identify and Compare Given Compounds

The given acids are: (a) acetic acid (\(\mathrm{CH}_{3} \mathrm{COOH}\)), (b) formic acid (\(\mathrm{HCOOH}\)), (c) isobutyric acid (\(\left(\mathrm{CH}_{3}\right)_{2} \mathrm{CH}-\mathrm{COOH}\)), and (d) an incorrect formula that should be checked as acetic acid with an ethyl group which does not exist correctly in the form given. We focus on comparing acetic acid, formic acid, and isobutyric acid because these are recognizable acids.

Compare Known pKa Values

Based on known chemistry data, the pKa values are: formic acid ~3.75, acetic acid ~4.76, and isobutyric acid ~4.84. A brief evaluation of these values shows that formic acid has the lowest pKa.

Conclusion on Lowest pKa Value

Since formic acid has the lowest pKa value among the given choices, it is the strongest acid among the options provided.

Key concepts

Understanding Acid Strength

When we talk about acids and their strength, we are referring to how well an acid can donate a proton (H⁺) in a solution. The measure of this ability is reflected in the acid's pKa value. The lower the pKa value, the stronger the acid. This is because a lower pKa indicates a higher degree of ionization, or dissociation, meaning the acid gives off its proton more readily.
In practical terms, strong acids dissociate almost completely in a solution, producing a high concentration of hydrogen ions. This increased ionization in solution translates to a lower pKa value because there's more separation of charges. Therefore, an acid’s strength can be directly compared through its pKa value, assisting chemists in determining which acids are more reactive in different scenarios.
So, if you're ever looking at a list of acids and trying to determine which one is stronger, just look at their pKa values – the lower, the stronger.

Characteristics of Organic Acids

Organic acids are a category of acids that contain carbon atoms in their structure in addition to the acidic hydrogen. These compounds are common in both the chemistry lab and the everyday world, present in vinegar, citrus fruits, and more.
These acids often contain the carboxyl group (COOH). The hydrogen in this group is what gives organic acids their acidic properties. When this hydrogen is released, the acid can donate a proton and act as an acid. Organic acids are generally weak acids, meaning they do not fully dissociate in solution. This attribute, however, does not mean they are not important; they play vital roles in metabolism and industrial applications.

• Examples include acetic acid (found in vinegar), formic acid (standard in ants), and citric acid (found in citrus fruits).
• Their strength, like all acids, can be compared using pKa values.

Understanding organic acids and their behavior helps in gauging their reactivity and applications in real-world contexts.

Acidity Comparison Among Acids

When comparing the acidity of different acids, pKa values serve as a reliable indicator. For instance, in comparing acetic acid, formic acid, and isobutyric acid, as seen in the original exercise, the pKa values tell us about their relative strengths.
Acetic acid (\(\mathrm{CH}_{3}\mathrm{COOH}\)) has a pKa around 4.76. It isn't the weakest acid, but it’s not the strongest either. Formic acid (\(\mathrm{HCOOH}\)) comes with a pKa around 3.75 and is stronger than acetic acid, as shown by its lower pKa, indicating better ionization and more proton donation than acetic acid.
On the other hand, isobutyric acid (\(\left(\mathrm{CH}_{3}\right)_{2} \mathrm{CH}-\mathrm{COOH}\)) has a pKa around 4.84, making it weaker than the others in this specific group.
Comparing these acids by their pKa allows you to determine easily which compound would be more "acidic" in a competitive scenario, helpful not just for chemistry exercises, but also for practical applications where acid choice matters – such as in chemical synthesis or product formulation.