# Solving pH of an Aqueous Solution of Weak Acid HCOOH

• geffman1
In summary, the conversation discusses a question about calculating the pH of a solution of a weak acid with a given concentration and Ka value. The person asks for help with writing the equilibrium expression and receives clarification on the correct Ka value. The conversation also mentions checking the calculation of pH.

## Homework Statement

hey guys, I've got a prob, i kinda know how to do this question but i can't get past the first step. i want to write an equilbrium expression by I am not sure what the chemical reaction is. any help would be good

## Homework Equations

A solution of a weak acid, HCOOH, is made to a concentration of 0.689 mol / L. Calculate the pH of this solution.
Ka of HCOOH is 1.8 x 10-4.

## The Attempt at a Solution

The reaction of acetic acid in water (after actually dissolving) is a neutralization reaction. The water acts as a base. Also, for writing the Ka expression, the concentration of water does not effectively change. Since it is essentially constant, it is not written as part of the Ka expression.

Your reported value for Ka is one magnitude wrong. It should be 1.8*10^(-5), which is ten times smaller than the value you showed.

Last edited:
Actually question is about formic acid and Ka value of 1.8x10-4 is correct.

Check calculation of pH of a weak acid lecture.

Borek said:
Actually question is about formic acid and Ka value of 1.8x10-4 is correct.

Check calculation of pH of a weak acid lecture.

Very Good. I understand.