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# Calculating the pH of a weak acid

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Given the concentration of a weak acid, calculate the pH of the solution. The Ka for acetic acid, CH3COOH, is 1.8x10^-5. Calculate the [H+] of a 0.20 M solution of acetic acid in water.

##### Solution Summary

In this solution I show you step by step (with a total of 6 steps), working out every step in detail, how to calculate the pH of this solution. We start off by filling out a table outlining initial concentrations, changes in concentrations, and final concentrations. Then, we develop a mathematical equation based on these values and the equilibrium expression. Finally, we utilize the quadratic formula to solve the expression and relate that solution to the concentration of H+ that we are looking for.

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CALCULATING THE pH OF A WEAK ACID

For acetic acid, CH3COOH, the Ka value is 1.8x10^-5. Calculate the concentration of H+ in a 0.20 M solution.

Step 1:

We were told that the initial concentration of CH3COOH was 0.20 M. Since no initial concentration values were given for H+ and CH3COO-, we assume that none was present. Therefore, we show this by placing a zero in the corresponding cells of the table below.

CH3COOH H+ CH3COO-
Initial Concentration 0.20 M 0 0
Change in Concentration
Equilibrium Concentration

Since we began without any concentration of H+ and CH3COO-, we know that as CH3COOH ionizes, it will produce some of both of these chemical species. Since we do not know how much of these ...

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