Dilution question

[unsung]

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789. An acid is added to water to make 30 mL of a solution with a pH of 6.5. If the solution is then diluted to a total volume of 300 mL, what will be the new value of the ph?

A. 6.5
B. Between 6.5 and 7
C. 7.5 because the hydrogen ion concentration is reduced by a factor of 10.
D. Either A or B depending upon what acid was used, but C is impossible.

The answer is D. I don't get how to do these Qs. 🙁

 

[161927]

Water at standard temperature cannot turn an acidic solution basic. That is the basis for eliminating option C where pH > 7. Before dilution, the pH is 6.5, and the addition of water may reduce the acidity up until the solution is neutral. A range of 6.5-7 is possible.

 

[161927]

This is also known as the "leveling effect"

 

[unsung]

Water at standard temperature cannot turn an acidic solution basic. That is the basis for eliminating option C where pH > 7. Before dilution, the pH is 6.5, and the addition of water may reduce the acidity up until the solution is neutral. A range of 6.5-7 is possible.

why does dilution reduce acidity if pH = -log [H+]? doesn't the concentration [H+] stay the same ?

 

[seraph524]

Concentration of a substance depends on how much volume there is, so dilution will decrease its concentration.

 

[unsung]

Concentration of a substance depends on how much volume there is, so dilution will decrease its concentration.

Ah... doh, yes. Thanks. So, how about this one: 0.1 M H2CO3 is being titrated with 0.1 M NaOH, what is [CO32-] at 2nd equivalence pt ? (it took 60 mL of 0.1 M NaOH to reach 2nd equivalence pt)

The answer is supposed to be 0.033 M because of dilution, but I don't know where that came from.

 

[nikeshp]

Concentration of a substance depends on how much volume there is, so dilution will decrease its concentration.

then how can the answer be D? A shouldnt be right

 

[Kaustikos]

then how can the answer be D? A shouldnt be right

Because it could also be a difference in the strength of the acid/tendency to give the proton in the solution. The acid could not dissociate at all, regardless of the volume of H2O added.