Acid and Water

[G1SG2]

Okay, I just want to double check something. If you add water to an acid (or rather, add acid to water ), you increase the percent dissociation of the acid:

HA+H20--> A- + H3O+

However, the pH doesn't decrease; it increases because all species are being diluted:

K=[H3O+][A-]/[HA]

Is this correct? One would think that the addition of water would decrease the pH because more solvent means more room for dissociation...but since everything is diluted, the solution experiences an increase in pH?

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[loveoforganic]

Sounds right to me. There would be more hydroniums in solution, but there's more solution.

 

[G1SG2]

Sounds right to me. There would be more hydroniums in solution, but there's more solution.

Gotchya, thanks!

 

[kobe200LATE]

Actually, I think because water isn't included in the acid dissociation constant, Le Chatelier's principle doesn't apply when you add more water. So because theres so much water and the concentration of water doesn't change, adding more water wouldn't favor the forward reaction.

However, it would just increase the volume of all of the species involved in the acid dissociation constant, decreasing the concentration and the Ka, increasing the pH

 

[G1SG2]

Okay, so what would your answer to this question be:

If a solution with a pH of 11 is diluted with pure water until the volume is increased by a factor of 10, the pH will then equal:

A. 1
B. 9
C. 10
D. 12

 

[loveoforganic]

B.

Just to note - any insoluble salt (acids and bases included) will dissociate to a larger percent when diluted to a greater extent.

 

[G1SG2]

B.

Just to note - any insoluble salt (acids and bases included) will dissociate to a larger percent when diluted to a greater extent.

Why B? The answer is actually C. Something about the pH moving toward neutrality because of dilution?

 

[loveoforganic]

Sorry, I misread. C is right.

 

[sv3]

Okay, so what would your answer to this question be:

If a solution with a pH of 11 is diluted with pure water until the volume is increased by a factor of 10, the pH will then equal:

A. 1
B. 9
C. 10
D. 12

Is the answer C b/c pure water has a pH of 7, thus brining the pH down?
On the other hand, let's say your solution had a pH of 5, then would this dilution make the final pH 6? (since water will now be less acidic, makin the solution more basic?)
I just want to make sure my thought process is right. I used to love chemistry but now the more i think about it the more my head comes close to exploding!

Sv3

 

[Red Leader]

Okay, so what would your answer to this question be:

If a solution with a pH of 11 is diluted with pure water until the volume is increased by a factor of 10, the pH will then equal:

A. 1
B. 9
C. 10
D. 12

C. log = factor of 10. If you are above pH 7, and dilute with water 10x you will decrease 1 pH value. If you are under pH 7 and dilute with water your pH will increase 1 pH value.

sorry to be more clear pH is based on the log scale.

so to take this example further, if you dilute something 100 times (10^2) with water, the pH will go up or down by 2 pH units and so forth.

 

[CaliforniaKid]

Indeed "Red Leader" is correct.

Think of it in terms of log scale (because pH is a log10 scale)

A factor of 10 means:
log(10) = 1
thus, you are increasing by 1 (on the log scale) and the pH drops from 11 to 10 (by 1).

if it was 100

then log 100=2

pH would be 9

 

[loveoforganic]

Keep in mind, this trend is an approximation, and you will not ever change from acidic to basic or basic to acidic via dilution.

 

[sv3]

Keep in mind, this trend is an approximation, and you will not ever change from acidic to basic or basic to acidic via dilution.

Would neutrality be the limit? I'm thinking if you have a solution with pH 10, you could dilute it infinitly but only hit a min pH of 7 (since thats pH of pure water). Same theory for acidic to basic applies.........or so i think?

 

[loveoforganic]

Yep, neutrality is the limit you would approach, but never reach.

 

[G1SG2]

Okay, so a weak base can completely neutralize a strong acid if enough is added, and vice versa. BUT we can end up with a solution that's slightly acidic or basic, depending upon what we end up with, right? For example, suppose we react equal/stoichiometric amounts of calcium hydroxide with carbonic acid:

H2CO3 + Ca(OH)2--> 2H2O + CaCO3

We would end up with a neutral solution because CaCO3 would precipitate out/it's not soluble. BUT, if we ended up with something soluble, like sodium carbonate or something for example, we can't say the solution is neutral because it will be slightly basic due to the presence of carbonate?

 

[loveoforganic]

correct

 

[sv3]

I think you need to look at the conjugates of the substances you have in the end. That should help determine acidity/basicity of final solution (in addition to whatever quant. info is available). I remember this coming in handy on titration problems. So if the conjugate of X is a weak acid, then X will contribute to the solution's basicity.