add water to acid

[sdnstud]

---

Members don't see this ad.

When water is added to an acidic aqueous solution:


A. the percent dissociation increases.
B. the proton concentration increases.
C. the acid strength increases.
D. the hydroxide ion concentration increases.

 

[Swack]

Heh, where did you get that question? You never add water to acid .

I think it's D.

 

[mdsquared]

sdnstud, what is the correct answer that is listed? And is there an explanation?

 

[Medikit]

The answer is A.

The question appears to assume you are dealing with a Br?nsted-Lowry Acid.

When water is added to an acid the proton associated with acid molecules become associated with water molecules. The acid has become dissociated from its proton and so the percent dissociation has increased.

D is wrong because hydroxide is formed when a proton dissociates from water, here protons are associating with water. Hydronium and the conjugate base of the acid are formed, not hydroxide. It is assumed that the acid in question is not water.

 

[cluelesspremed]

The answer is A.

The question appears to assume you are dealing with a Br?nsted-Lowry Acid.

When water is added to an acid the proton associated with acid molecules become associated with water molecules. The acid has become dissociated from its proton and so the percent dissociation has increased.

D is wrong because hydroxide is formed when a proton dissociates from water, here protons are associating with water. Hydronium and the conjugate base of the acid are formed, not hydroxide. It is assumed that the acid in question is not water.

So the acid's going to dissociate into H+ and A-, but because the water also can ionize to both H+ and OH- I don't see why you don't have to take into account that there'll be additional H+ going into solution and the whole common ion effect...? 😕

 

[anitra627]

Why wouldn't the anwer be B?

HA + H20------> H+ + A-

So doesn't adding water, shift the reaction to the right? Or is that wrong because we don't know that the reaction is in equilibrium? Does it matter if it's in equilibrium anyway?

I HATE acid base chemistry.

 

[BisMuOxo]

The equilibrium only depends on concentrations. The concentration of H2O is essentially the same even when you add water.

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

Adding water will dilute the concentrations of all species. So the denominator decreases but both terms in the numerator decrease.

B is incorrect because all concentrations will decrease.
C is incorrect because K doesn't change in a given solvent.
D is incorrect because
For example lets say KA=1 and the concentrations of H+, A- and HA are all 1M

K=(1)(1)/(1)=1

Now lets say you double the volume of water (cutting all of the concentrations in half)
Now, before the system goes to equilibrium you have all species at 0.5M so the reaction quotient is

Q=(0.5)(0.5)/(0.5)=0.5

When the reaction quotient is smaller than the equilibrium constant (recall from gen. chem) the reaction must go toward products to reach equilibrium. Hence, more HA dissociates to form H+ and A- untill the reaction quotient is 1. A larger percent of HA has dissociated.

 

[anitra627]

Fantastic. Thank you!

 

[Swack]

The answer is A.

The question appears to assume you are dealing with a Br?nsted-Lowry Acid.

When water is added to an acid the proton associated with acid molecules become associated with water molecules. The acid has become dissociated from its proton and so the percent dissociation has increased.

D is wrong because hydroxide is formed when a proton dissociates from water, here protons are associating with water. Hydronium and the conjugate base of the acid are formed, not hydroxide. It is assumed that the acid in question is not water.

Of course...formation of the hydronium ions will decrease the amount of free protons, thus shifting acid dissociation to the right 👍. Hmm..I forgot why I thought it was D.

 

[chopper]

Are there every any free protons? I thought it was always H30+, we just say H+ as a short hand?

 

[Medikit]

Of course...formation of the hydronium ions will decrease the amount of free protons, thus shifting acid dissociation to the right 👍. Hmm..I forgot why I thought it was D.

Maybe because the pH was increasing and we associate high pH's with hydroxide ions.

 

[shaq786]

Maybe because the pH was increasing and we associate high pH's with hydroxide ions.

Yea thats what had got me. I guess it is A. But here is my question...How do we change Ka value for a certain acid?

I know one way is through increasing the temperature, but what else can we do to increase Ka for an acid?