The Ka values of maleic acid, HOOCCHCHCOOH, are shown below.
Ka1 1.5 10-2
Ka2 2.6 10-7
A 50.0 mL sample of 0.11 M of maleic acid is titrated with 0.14 M NaOH. Calculate the pH at the following points in the titration curve.
find the first and second eq point pH
i thoguth i was doing it right but i cant get it
For the first equivalence point,
(H^+) = sqrt(k1k2)
For the second equivalence point, pH is determined by the hydrolysis of the salt, in this case Na2M
M^-2 + HOH ==> HM^- + OH^-
Write the Keq (Kb) expression, set up an ICE chart, solve for OH^-, convert to pOH, then to pH.
To calculate the pH at the first and second equivalence points in the titration curve of maleic acid with NaOH, we need to understand the concept of equivalence point and how to determine it.
In a titration, the equivalence point is the point at which stoichiometrically equivalent amounts of the acid and base have been reacted. At the equivalence point, the moles of acid equal the moles of base, resulting in a neutral solution.
For maleic acid (HOOCCHCHCOOH), it is a diprotic acid, meaning it can donate two protons (H+ ions) per molecule. Therefore, it will have two equivalence points.
Let's calculate the first and second equivalence point pH step by step:
1. First, find the moles of maleic acid in the initial 50.0 mL sample:
Moles of maleic acid = volume (in liters) x concentration
Moles of maleic acid = (50.0 mL / 1000 mL/L) x 0.11 M
2. Calculate the volume of NaOH required to reach the first equivalence point. At the first equivalence point, all the maleic acid has reacted with an equal number of moles of NaOH. Since maleic acid is diprotic, two moles of NaOH will react with one mole of maleic acid:
Moles of NaOH at first equivalence point = 2 x moles of maleic acid
Volume of NaOH at first equivalence point = (moles of NaOH / NaOH concentration)
3. Calculate the concentration of maleic acid remaining at the first equivalence point:
Initial concentration - concentration used at the first equivalence point
4. Find the concentration of the maleate ion (the maleic acid after losing its first proton) at the first equivalence point. This can be calculated using the balanced equation:
Maleic acid (C4H4O4) + NaOH → Maleate ion (C4H3O4-) + H2O
The stoichiometry tells us that one mole of maleic acid reacts to produce one mole of maleate ion. Therefore, the concentration of maleate ion at the first equivalence point will be the same as the concentration of maleic acid remaining.
5. Use the Ka value (Ka1) of the maleate ion to calculate the pH at the first equivalence point. The formula to use is:
Ka1 = [H+][Maleate ion] / [Maleic acid - Maleate ion]
Repeat the same steps to calculate the second equivalence point pH using Ka2 of maleic acid at this point.
By following these steps, you should be able to calculate the first and second equivalence point pH for the given titration.