chemistry. pH question. help?

Question

when 10 mL .10M H2SO4 is added to 40 mL .10M NaOH, the pH changes about .5 units. After 10 more mL H2SO4 is added, the pH changes about 6 units. Why?

to anyone who can help, i will be eternally greatful :)

Answer 1

Initially, you have 1 millimole of H2SO4 reacting with 4 millimoles of NaOH. This has the effect of creating 2 millimoles of Na2SO4, and 2 millimoles remain of OH-. Since this in in 5/4 the original solution volume, the molar concentration decrease is 2 x 5/4 = or 2.5. This means the OH- level had dropped to 1/(2.5)th of its initial level. The initial level was 0.1 M, now it is 4x10^-2. The pH level is 2.5x10-13, which is a 0.4 or so decrease in pH. The initial pH was 13, now it is about 12.6.

When the other part of the H2SO4 is left, you effectively have water with Na2SO4, which is a neutral salt (strong base-strong acid). So the pH is down around 7.

Answer 2

The first addition of acid did not cause a very large percentage change in the overall amount of base. The second addition caused a much greater percentage change, hence the greater shift in pH.

If one plots the pH versus volume of the titrant (the H2SO4 in your case), there is a very small pH change, then within a very few mL (even drops right around the equivalence point) there is a large pH change. This is called a titration curve and tends to take on an elongated S shape.

Eternally grateful?? Hmmmm.

Answer 3

Because you have already diluted it and neutralized it