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Calculate Ka of a 0.10 M solution of lactic acid has a pH of 2.44

October 21, 2019

Lactic acid has one acidic hydrogen. A 0.1 M solution of lactic acid has a pH of 2.44. Calculate Ka for lactic acid? When we do acid/base questions, we always need to ask ourselves 2 questions.

Question1 Do we have an acid or a base?

In this case the question itself, told us that we are dealing with an acid.

Question 2 Is this a weak or a strong acid?

Strong acids are: HCl - hydrochloric acid HNO3 - nitric acid H2SO4 - sulfuric acid (HSO4- is a weak acid) HBr - hydrobromic acid HI - hydroiodic acid HClO4 - perchloric acid HClO3 - chloric acid

Our acid does not belong to this list and is therefore a weak acid.

 

For weak acids, we must always do ICE charts. Before the ice chart, we should write the reaction. Generic reaction of dissociation of a weak acid is HA -- H+ + A- We plug all the values in our ice chart to see the equilibrium values, which are .1-x, x and x. Since we are given the pH, we can calculate [H+], since it is equal to 10^-pH. This calculation gives us the equilibrium concentration of H+ or x. Plugging our value into the x, we can easily calculate Ka.

 

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