[777888]

1) What is the meaning of Ka and pKa? The formula is pKa=-logKa , but what does it mean?

2)In titration, what is the difference between equivalent point and end point?
[Equivalent point means # mol of acid=# mol of base, so for the titration of HCl+NaOH, the equivlent point has a pH of 7 because 1:1 mol ratio. Then would the pH still be 7 if the mol ratio is not 1 to 1? (like 1:2 mol ratio in balanced chemical equation?!)

THANKS

[Donaldson Tan]

pKa = -lg Ka. p is just a mathematical function.

equivalent point is the actual pH where your titration should end. However, the indicators we used only change colour near the equivalent point. The pH whereby our indicators change colour is call the end point.

[777888]

pKa = -lg Ka. p is just a mathematical function.

equivalent point is the actual pH where your titration should end. However, the indicators we used only change colour near the equivalent point. The pH whereby our indicators change colour is call the end point.

Oic! Thanks!

So does equialence point always have a pH of 7? WHY?

[777888]

HF is titrated using NaOH

HF + NaOH -> NaF + H2O

Why at midpoint(halfway to eqvilvance point), it is the HF that undergoes hydrolysis instead of F-?

[777888]

HF is titrated using NaOH

HF + NaOH -> NaF + H2O

Why at midpoint(halfway to eqvilvance point), it is the HF that undergoes hydrolysis instead of F-?

HF is titrated using NaOH

Also, why at midpoint, pH=pKa when weak acid is titrated with strong base? (used for checking in calculations)

[777888]

I still don't get what the equivlance point is. The definition is when mol of acid=mol of base. But what if the acid-base reaction is not in 1:1 mol ratio?
Say H2A + 2BOH -> H2O+ BA (The reactant (acid-base) side has 1:2 mol ratio)
Note: I didn't balance the right side.

Also, in weak acid/strong base titration, the equilvalence point doesn't have a pH of 7. So do we call that it is neturalized?

[Donaldson Tan]

equivalence point is the point when titration reaction is complete.

the end point is the point when the indicator used changes colour.

for accurate titration, equivalence point must equal end point. However, this is rarely the case due to technical limitations.

pH of a complete titration isn't necessary 7 at 25C, especially when the product mixture contains ions that hydrolyses to produce H+ or OH- in solution, eg. the pH of a ethanoic acid titrated with sodium hydroxide will > 7 because the sodium ethanoate formed hydrolyses water to form OH-.