Remember acid and bases have weak and strong properties in A-Level Chemistry?
Through titration we can look at their pH curve and determine whether they are strong or weak!
Titration:
1) add a measure of acid (with known concentration) to burette
2) rough titration; swirl conical flask for approximate end point*
3) accurate titration; drop by drop
4) record amount of base needed to neutralise the acid
5) Repeat for more accurate readings
*end point: when the solution changes colour (also known as equivalence point – see below)
Equivalence point
where a tiny amount of alkali causes a sudden big change in pH, where the acid is JUST neutralised. Equivalence point will vary depending on acid/alkali used. For the last graph below between a weak acid and weak alkali, a pH meter is the best thing to use to find the equivalence point as the colour change is gradual and unclear.
pH curve:
CALCULATION:
finding the Ka of a weak acid using a pH curve
HA <--> H+ + A-
The half equivalence point is the point where half the acid has been neutralised, where half the volume of strong base has been added to the weak acid before equivalence.
At the half equivalence point [HA]= [A-]
Therefore;
Ka= [H+] [A-] / [HA]
Ka= [H+]
pKa= pH
Thus, we can say that the half equivalence point is also the pKa of the weak acid, then we can use Ka = 10-pKa
Drafted by Eunice (Chemistry)