# Identifying a Weak Acid: Solving for pH in a 0.50 mol/L Solution - Homework Help

• Euler2718
In summary, the question asks for the possible identity of a weak acid with a pH of 3.18 in a 0.50 mol/L solution. After using the ICE table and inverse log formula, the Ka expression for the acid was calculated to be 8.75 x 10^-7. The closest acid with that Ka value is carbonic acid, although its pKa still does not align perfectly with the given information.
Euler2718

## Homework Statement

What is the likely identity of a weak acid if a 0.50 mol/L solution of the acid has a pH of 3.18?

## Homework Equations

HA + H2O <=> H3O + A

$$K_{a} = \frac{[P]}{[R]}$$

$$[H3O] = 10^{-pH}$$

## The Attempt at a Solution

[/B]
I set up an ICE table with the concentration given, then solved for x using the inverse log formula for [H3O], finally I did the following Ka expression:

$$K_{a} = \frac{(6.6 \times 10^{-4})^{2}}{0.5-6.6 \times 10^{-4}} = 8.75 \times 10^{-7}$$

The closets acid to that appears to be carbonic acid (ka = 4.4x10^-7). Any suggestions?

You've doubly ionized a diprotic weak acid at a pH of 3.2? Try it again.

Bystander said:
You've doubly ionized a diprotic weak acid at a pH of 3.2? Try it again.

I am not sure what you mean, Ka is calculated assuming the acid behaves as a monoprotic. That's not a bad assumption for a carbonic acid (no idea if the OP was aware of the problem, but that's another story).

Sadly, pKa of 6.05 doesn't fit any acid commonly used in introductory chemistry courses. Of those used carbonic acid looks best (but still is way off with the pKa1=6.37).

Euler2718

## 1. What is the definition of a weak acid?

A weak acid is a type of acid that does not completely dissociate in water. This means that only a small portion of the acid molecules will release hydrogen ions into the solution, resulting in a low concentration of hydrogen ions and a higher pH compared to strong acids.

## 2. How is the strength of a weak acid determined?

The strength of a weak acid is determined by its dissociation constant, also known as Ka. This value represents the equilibrium constant for the reaction of the weak acid with water and indicates the extent to which the acid dissociates in solution.

## 3. What are some examples of weak acids?

Some common examples of weak acids include acetic acid (found in vinegar), citric acid (found in citrus fruits), and carbonic acid (found in carbonated beverages). Weak acids are also commonly found in biological systems, such as lactic acid in muscles and amino acids in proteins.

## 4. How does a weak acid differ from a strong acid?

A weak acid differs from a strong acid in its ability to dissociate in water. Strong acids fully dissociate, meaning that all of the acid molecules release hydrogen ions into the solution. In contrast, only a small percentage of weak acid molecules will dissociate, resulting in a lower concentration of hydrogen ions and a less acidic solution.

## 5. How is the pH of a weak acid solution calculated?

The pH of a weak acid solution can be calculated using the Henderson-Hasselbalch equation, which takes into account the dissociation constant, concentration of the weak acid, and the concentration of its conjugate base. Additionally, the pH can be measured using a pH meter or calculated using the concentration of hydrogen ions in the solution.

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