Dilution

[Abdul Quadeer]

1. The problem statement, all variables and given/known data

Multiple Choice-

In which of the following solutions effect of dilution will not take place??

(A) 100 ml 0.1 M CH3COOH with 50 ml 0.1 M NaOH

(B) 100 ml 0.1 M CH3COOH with 100 ml 0.1 M NaOH

(C) 100 ml 0.1 M CH3COOH with 50 ml 0.1 M NH4OH

(D) 100 ml 0.1 M CH3COOH with 100 ml 0.1 M NH4OH


3. The attempt at a solution

I think the answer is B and D because the solutions will be neutralised and there won't be any effect on dilution.

But the answer given is A and D.

Help

[Borek]

No idea what the question asks. Dilution of what? Effect on what?

[Abdul Quadeer]

Dilution of the solution which is obtained after mixing the liquids in given options.
Effect on pH of the solution.

[Borek]

Question still doesn't make sense to me. If you mix solution of acid with solution of base, final pH will be always different from pH of original solutions.

So far question asks to compare apples and oranges.

[Abdul Quadeer]

Look- after mixing the two solutions, let the pH of the resulting solution be x
Now on adding water, will the pH be x or different?

I hope you understood.

[Borek]

OK, got it. Not necessarily the best wording, I was all the time thinking about dilution that takes place when solutions are mixed.

Not only wording is poor, also answer is incorrect. Usually I would help you to solve the question on your own, but I have no idea what the expected answer is.

The only solution that has constant pH is the last one - that is, ammonium acetate.

Solution A: pH after mixed 4.67, after diluted with 100 mL of water - 4.69
Solution B: pH after mixed 8.63, after diluted with 100 mL of water - 8.56
Solution C: pH after mixed 4.67, after diluted with 100 mL of water - 4.69
Solution D: pH after mixed 7.00, after diluted with 100 mL of water - 7.00

You may have troubles getting these numbers, as they take ionic strengths into account (I have calculated them using this pH calculator (http://www.chembuddy.com/?left=Buffer-Maker&right=pH-calculator)). When ionic strengths are ignored calculated pH values are:

Solution A: pH after mixed 4.76, after diluted with 100 mL of water - 4.76
Solution B: pH after mixed 8.71, after diluted with 100 mL of water - 8.63
Solution C: pH after mixed 4.76, after diluted with 100 mL of water - 4.76
Solution D: pH after mixed 7.00, after diluted with 100 mL of water - 7.00

A & C are buffers, so their pH is more or less constant. Solution B is a solution of salt of a weak acid, which is equivalent to solution of a weak base, its pH is obviously dependent on the concentration. Solution D is a funny case of solution of a salt of a weak acid and a weak base - funny, as they are both weak in exactly the same degree, so effects of their hydrolysis cancel out.

[Abdul Quadeer]

Solution D: pH after mixed 7.00, after diluted with 100 mL of water - 7.00

Even after mixing water, the pH is constant right? then why D is not a correct option?

[Borek]

D is correct, but if you have to select correct ones, A & D are not all that are correct - A, C & D are. Hence A & D is not the correct answer.

--
buffer calculator (http://www.chembuddy.com/?left=Buffer-Maker&right=buffer-calculator), concentration calculator (http://www.chembuddy.com/?left=CASC&right=concentration_and_solution_calculator)
pH calculator (http://www.chembuddy.com/?left=BATE&right=pH-calculator), stoichiometry calculator (http://www.chembuddy.com/?left=EBAS&right=equation-balancing-stoichiometry)

[Abdul Quadeer]

Thanks Mr.PH!!