EK 1001 Chem: Acid Diss. of Water Question

[Vanguard23]

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#771. What is the acid dissociation constant Ka of water?
A. 1.0x10-14
B. 1.8x10-16
C. 1
D. 10

I chose A. It seemed at least reasonable, even if I knew it was technically wrong. The actual answer? B. Rationale?
-The Ka is just Kw divided by 55 the concentration of water (55 mol/L)

What the hell? This is a stand alone question; no passage attached to it. I didn't know the concentration of water was relevant to the *acid* dissociation constant. New on me.

 

[Bernoull]

#771. What is the acid dissociation constant Ka of water?
A. 1.0x10-14
B. 1.8x10-16
C. 1
D. 10

I chose A. It seemed at least reasonable, even if I knew it was technically wrong. The actual answer? B. Rationale?
-The Ka is just Kw divided by 55 the concentration of water (55 mol/L)

What the hell? This is a stand alone question; no passage attached to it. I didn't know the concentration of water was relevant to the *acid* dissociation constant. New on me.

I'm interested in this answer, I can see how B can be correct but I though concentrations of pure solids n liquids are excluded from the Equilibrium expression so 55mol/L for pure water should be excluded??😕😕😕

 

[Dr Gerrard]

http://forums.studentdoctor.net/showthread.php?p=7027038

There is the same question.

I just don't get why you include [H20] in the Ka = [H][A]/[HA], when in this case, HA = H20 = a liquid.

http://en.wikipedia.org/wiki/Acid_dissociation_constant#Water_self-ionization

I guess because Kw is when the water concentration is said to be constant. It is a special version of Ka.