500 cm3 of a buffer solution of pH = 4.70 is required. Calculate the volume of 0.800 mol dm–3 sodium ethanoate solution and of 0.800 mol dm–3 ethanoic acid needed to make this buffer. [Ka] (3)
I don’t understand what you do after calculation the concentration of H+ ions. Can someone explain? Thanks (I looked at the mark scheme and still don’t get it)
500 cm3 of a buffer solution of pH = 4.70 is required. Calculate the volume of 0.800 mol dm–3 sodium ethanoate solution and of 0.800 mol dm–3 ethanoic acid needed to make this buffer. [Ka] (3)
I don’t understand what you do after calculation the concentration of H+ ions. Can someone explain? Thanks
After calculating the H^+ ion concentration, you can use Ka = [H^+][A^-]/[HA] to find the ratio of A^- to HA in the buffer solution.
The ratio of [A^-] to [HA] in the buffer solution, [A^-]/[HA], is simply found by dividing both sides through by [H^+]:
[A^-]/[HA] = Ka/[H^+]
You can use the fact that concentration is directly proportional to moles to deduce that this ratio is also equal to the mole ratio of A^- to HA.
You can then say A^- and HA are in an n:1 ratio, so [A^-]/[HA] = n/1. Once you have this ratio, split the 500 cm^3 into this ratio, since the solutions used are of the same concentration. That should tell you the volumes of each solution needed.
After calculating the H^+ ion concentration, you can use Ka = [H^+][A^-]/[HA] to find the ratio of A^- to HA in the buffer solution.
The ratio of [A^-] to [HA] in the buffer solution, [A^-]/[HA], is simply found by dividing both sides through by [H^+]:
[A^-]/[HA] = Ka/[H^+]
You can use the fact that concentration is directly proportional to moles to deduce that this ratio is also equal to the mole ratio of A^- to HA.
You can then say A^- and HA are in an n:1 ratio, so [A^-]/[HA] = n/1. Once you have this ratio, split the 500 cm^3 into this ratio, since the solutions used are of the same concentration. That should tell you the volumes of each solution needed.
What do you mean divide both sides by [H+]
This is the formula: Ka=[H+][A-]/[HA] so what do u divide by [H+]? At least use some numbers, I already said I don’t understand it.
So I divide Ka by [H+] but then how do I divide the other side if it’s [H+][A-]/[HA]?
Ok thanks so much. This made me understand it a lot better. 1 question: does diving Ka by [H+] give you the [A-]? And if so why?
Dividing Ka by [H^+] gives you the ratio of A^- to HA, rather than giving you just [A^-]. It’s because it is always assumed that the ratio can be written in the form n : 1 and it’s just easier to make the HA part of the ratio 1, since it is on the bottom of the fraction [A^-]/[HA].
The ratio of A^- to HA is conveniently what you need for finding the volumes of each solution required.