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Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: seerrah_ on April 12, 2019, 03:54:48 PM

Title: Calculate amount of acid needed in a solution to lower pH
Post by: seerrah_ on April 12, 2019, 03:54:48 PM

Hi,

I can't figure out the following problem..

When a 30mM (aq) solution of sodium acetate was prepared in the lab, it had a pH of 8.8. Calculate the amount of acetic acid (in grammes) to be added to 0.5L of this sodium acetate solution in order to bring the pH to the target value of 5.00.

Any help would be greatly appreciated!!

Title: Re: Calculate amount of acid needed in a solution to lower pH
Post by: AWK on April 12, 2019, 05:18:50 PM

Addition of acetic acid forms a buffer solution. Amount of the acetic acid is so small that you can safely neglect the volume change in calculations.

Title: Re: Calculate amount of acid needed in a solution to lower pH
Post by: seerrah_ on April 13, 2019, 05:19:27 AM

I’m still none the wiser if anyone could help?

Title: Re: Calculate amount of acid needed in a solution to lower pH
Post by: AWK on April 13, 2019, 08:54:54 AM

Calculate the ratio of acid to salt concentration from pH 5.0 (using the Henderson-Hasselbach equation). Then scale the problem down from 1 L to 0.5 L and calculate the mass of acetic acid. It is a really simple problem.