Acid/base/pH problem

[549]

Passage talks about chlorinating water in swimming pools, and gives the following reaction equation:

Cl2(g) + H2O <> HCl (aq) + HOCl (aq), Keq=(1E-3)

Q:If the pH of swimming pool water is 5, what is the concentration of HOCl?

A: If the pH is 5, then the [HCl] must be (1E10-5), and this equals the [HOCl] as well because they are formed in a 1:1 ratio.

I attempted to answer this question a little differently though. I used the Keq given, and used it as the dissociation constant for HClO <> H+ + ClO- and calculated the [HOCl] needed to have produced pH 5, if all the H+ ions were from HOCl. Since this gives a value of [HOCl] much larger than what it actually would have been if I had used the real Keq for the reaction, I chose the only answer that was smaller than what I calculated, which was incorrect.

While I understand the answer given, I don't understand where exactly my logic fails, though clearly it does! Any help appreciated

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[vsl5]

Why would you use the Keq given and apply it to a completely different reaction? That is clearly wrong. HCl is a much stronger acid than HClO, so assume that all the protons produced can be attributed to HCl. [HCl] before dissociation = [HOCl] because the source of both is Cl2 and water.

Passage talks about chlorinating water in swimming pools, and gives the following reaction equation:

Cl2(g) + H2O <> HCl (aq) + HOCl (aq), Keq=(1E-3)

Q:If the pH of swimming pool water is 5, what is the concentration of HOCl?

A: If the pH is 5, then the [HCl] must be (1E10-5), and this equals the [HOCl] as well because they are formed in a 1:1 ratio.

I attempted to answer this question a little differently though. I used the Keq given, and used it as the dissociation constant for HClO <> H+ + ClO- and calculated the [HOCl] needed to have produced pH 5, if all the H+ ions were from HOCl. Since this gives a value of [HOCl] much larger than what it actually would have been if I had used the real Keq for the reaction, I chose the only answer that was smaller than what I calculated, which was incorrect.

While I understand the answer given, I don't understand where exactly my logic fails, though clearly it does! Any help appreciated

 

[549]

It wasn't that I was trying to apply the Keq value to a different equation (which I agree makes no sense)- I was trying to use it as a "benchmark." Anyway, I think my method just introduces too many estimations, and is probably why it doesn't work. Thanks anyway...

 

[Rabolisk]

I don't understand what you mean. Could you explain what you were trying to do?