Buffer solution help!!

Stuck on a buffer solution question
Q4.)
a.) Calculate the pH of a buffer solution formed by mixing 50.0 cm3 of 0.500 mol dm-3 ethanoic acid (Ka = 1.74 x 10-5 mol dm-3) with 10.0 cm3 of 0.800 mol dm-3 sodium hydroxide.

b) Calculate the pH of this buffer solution if 2.0 cm3 of 0.20 mol dm-3 hydrochloric acid is added.

Reply 1

TypicalNerd

18

Original post by hotchocolatecara

Stuck on a buffer solution question
Q4.)
a.) Calculate the pH of a buffer solution formed by mixing 50.0 cm3 of 0.500 mol dm-3 ethanoic acid (Ka = 1.74 x 10-5 mol dm-3) with 10.0 cm3 of 0.800 mol dm-3 sodium hydroxide.

b) Calculate the pH of this buffer solution if 2.0 cm3 of 0.20 mol dm-3 hydrochloric acid is added.

For part (a):

Start by writing down the Ka expression for ethanoic acid. Can you rearrange it to make [H^+] the subject?

Next, calculate the moles of ethanoic acid in 50 cm^3 of 0.500 mol dm^-3 ethanoic acid solution and the moles of sodium hydroxide in 10 cm^3 of 0.800 mol dm^-3 sodium hydroxide.

Using the equation:

CH3COOH + OH^- —> CH3COO^- + H2O

The moles of OH^- added to the solution (so the moles of sodium hydroxide) equal the moles of CH3COO^- ions produced in the reaction. Therefore, the moles of CH3COOH left is given by (moles of ethanoic acid calculated before) - (moles of sodium hydroxide calculated before)

Now using the moles of CH3COOH left and the moles of CH3COO^- ions produced, plug them into your rearranged Ka expression to find [H^+] (you can get away with using the moles of CH3COOH and CH3COO^-, rather than their concentrations). You should now be able to solve for the pH using pH = -log[H^+].

For part (b):

Hydrochloric acid is a strong acid that dissociates completely in the buffer solution, so it adds H^+ ions that react with some of the CH3COO^- ions to form more CH3COOH:

H^+ + CH3COOH —> CH3COOH

Calculate the moles of HCl in the 2.0 cm^3 of solution.

The new moles of CH3COOH in the buffer solution are found using (moles of CH3COOH left in part (a)) + (moles of HCl added) and the moles of CH3COO^- left in the buffer solution are found using (moles of CH3COO^- formed in part (a)) - (moles of HCl added).

You now finish this part of the question in the same way as you finished part (a).

Reply 2

Ðeggs

22

Looks like someone pasted a Chat GPT response? :lol:

Reply 3

TypicalNerd

18

Original post by Ðeggs

Looks like someone pasted a Chat GPT response? :lol:

Probably.

Besides, although the Henderson-Hasselbalch equation is a more efficient method of calculating the pH of a buffer solution, I’m not sure it’s considered acceptable below undergraduate level, so if the OP is asking an A level question, they should follow the method outlined in my post.

Original post by TypicalNerd

Probably.

Besides, although the Henderson-Hasselbalch equation is a more efficient method of calculating the pH of a buffer solution, I’m not sure it’s considered acceptable below undergraduate level, so if the OP is asking an A level question, they should follow the method outlined in my post.

Several pre-university courses (A' level, IB) do teach and expect knowledge of HH.
Personally, I feel that it is a potential source of confusion and always advise students to work directly with ka.

Buffer solution help!!

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