Question

Equal volumes of four 0.1 mol L
−
1
acids were titrated with the same sodium hydroxide
solution.
Which one requires the greatest volume of base to change the colour of the indicator?
(A)
Citric acid
(B)
Acetic acid
(C)
Sulfuric acid
(D)
Hydrochloric acid

Answer 1

You'll need more base for strong acids, less for weak acids.

Ka's are used to measure the "strength" (dissociation) of a weak acid, the greater this value is the more dissociation there is. (don't confuse it with pKa, which is the negative logarithm of the Ka, in which case a lower pKa value means a stronger acid.)

For the strong acids, the more acidic hydrogens it has, the "stronger" the acid is. (HINT: look at their formulas)

Answer 2

Ohh ok, so depending on the amount of hydrogen atoms, this will tell me how strong each acid is?

Answer 3

that would only work for the strong acids in the list above.

for weak acids, you can look at their Ka values.

you should first categorize them into whether they're strong or weak acids.

Answer 4

That's what i'm confused about. How do I work out which ones are stronger? Don't I have to know how to work out the pH or something?

Answer 5

you can't work out the pH of a solution if you don't know it's concentration.

when you think of the strength of an acid you have to think of it in terms of the stability of it's conjugate base.

for example, HCl -> H+ + Cl-

Cl- is a very stable conjugate base, because of the stable electron configuration it has and because Cl is very electronegative.

The strength of an acid is also determined by the strength of the bond of the acidic hydrogen. It's bond is also very weak because of the electron density polarization, which is mainly focused on the Cl.

This one
v
H - Cl

Answer 6

So hydrochloric acid is the strongest because Cl- has a strong conjugate base.. therefore needs the greatest volume of base to change the colour, am I right??

Answer 7

HCl is a very strong acid, but sulfuric acid, H2SO4, is stronger because of the stability of it's conjugate base and because it has 2 acidic hydrogens.

Answer 8

I seee, thank you thank you thank you!

Answer 9

no problem !