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PH curve quesiton

Can someone explain how to do this question?
Chemistry.jpg120.1KB
Reply 1
Avatar for Pigster
Pigster
20
How many marks is it? My guess is quite a handful.

Since you're not given the Ka value, you have to work it out. To do that, read off the volume of NaOH required to neutralise the HA, then read off the pH of the mixture when half that volume of NaOH was added. This pH = pKa and hence Ka is worked out.

Then, read off the pH when 0 cm3 of NaOH was added. From this you can work out [H+]. Now you have [H+] and Ka, so [HA] follows.
Original post by Pigster
How many marks is it? My guess is quite a handful.

Since you're not given the Ka value, you have to work it out. To do that, read off the volume of NaOH required to neutralise the HA, then read off the pH of the mixture when half that volume of NaOH was added. This pH = pKa and hence Ka is worked out.

Then, read off the pH when 0 cm3 of NaOH was added. From this you can work out [H+]. Now you have [H+] and Ka, so [HA] follows.


You just use the graph to find the volume required for neutralisation. Then M1V1 = M2V2
Reply 3
Avatar for Pigster
Pigster
20
Oh that's just too easy.

My way is way more fun.
What is the answer? I tried doing it and want to check if I got it right.
Original post by Alexis1729
What is the answer? I tried doing it and want to check if I got it right.


Show us your working
Because its a monoprotic acid, I thought that you would need equal moles to react with sodium hydroxide (because it only has one OH ion). Then I found that 11.8 cm3 of NaOH was used to neutralise the acid and multiplied 11.8 by 0.12 to find the total no. of moles used. I divided that answer by 25 to get 0.057 mol/cm3 as the concentration.

By the way, which syllabus is this question from?

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