Chemical Forums
Chemistry Forums for Students => High School Chemistry Forum => Topic started by: Shea on August 22, 2006, 06:50:27 PM

I need help with this question. It is not from an assignment. It is in my book, and it tries to explain how to do it, but I don't understand...
Determine the pH of a solution with a hydrogen concentration of 3.5 x 10^4
This is what it says.
pH = log [3.5 x 10^4]
= log 3.5  log 10^4
=  .54  (4)
=  .54 + 4
= 3.46
Where does the .54 come from?
And what Exactly does log mean in this?

Never mind, I got it now. I'm even more lost though on this.
Write an expression for K^{p} in the following:
2NO2 > N2O4
and
Write an expression for the ionic product of water K_{w}
and
Write an expression for K_{c} for the dissociation reaction of butanoic acid
H(C4H7O2) <> C4H7O2 + H+
I don't get the K^{p}, K_{c}, and K_{w}. The book doesn't explain this, even though it says on the cover that it makes chemistry easy to learn with full explanations...

And what Exactly does log mean in this?
http://en.wikipedia.org/wiki/Logarithm

I don't get the K_{p}, K_{c}, and K_{w}
http://en.wikipedia.org/wiki/Reaction_quotient
K_{p} uses pressures, K_{c} and K_{w} use concentrations. K_{w} is just K_{c} for water dissociation, although it is usually assumed that water concentration is constant.

Thanks for explaining that, but what do you think the questions mean by, "write an expression?"

Look at this page:
http://www.chembuddy.com/?left=pHcalculation&right=introductionacidbaseequilibrium#eq1.1
Equation 1.1 is the expression for the acid dissociation constant.

Um, that page wont open. When I click on it, it goes to a site that offers some highly questionable search topics...

Thanks for explaining that, but what do you think the questions mean by, "write an expression?"
This (http://en.wikipedia.org/wiki/Equilibrium_constant) wikipedia article should help explain the equilibrium constant.
The following is what you need to remember:
(https://www.chemicalforums.com/proxy.php?request=http%3A%2F%2Fupload.wikimedia.org%2Fmath%2F5%2Fa%2F8%2F5a8b078dcf05070c74af1ef5d3626158.png&hash=4bc26bd8e41a602b4dfb98c3206d38bb)(https://www.chemicalforums.com/proxy.php?request=http%3A%2F%2Fupload.wikimedia.org%2Fwikipedia%2Fcommons%2F5%2F5c%2FGleichgewicht.png&hash=91fa786ca10e001292eeea067ed63bfb)(https://www.chemicalforums.com/proxy.php?request=http%3A%2F%2Fupload.wikimedia.org%2Fmath%2F9%2F2%2Fd%2F92d799525c5bec2655bd431487cd27ac.png&hash=ecbff3ac87821cd09f0d6af11f393829)
and
(https://www.chemicalforums.com/proxy.php?request=http%3A%2F%2Fupload.wikimedia.org%2Fmath%2F8%2F2%2Fe%2F82e273f9e108ceaea60dd7f69a4ab244.png&hash=962d1183c8ce4c5f3a66b58ce910b4ab)
When they tell you to write an expression they want you to fill in the above expression (ignore k_{f} and k_{b}).
For K_{p} see this (http://en.wikipedia.org/wiki/Partial_pressure#Equilibrium_constants_of_reactions_involving_gas_mixtures). The expression is similar, just only include gases.
As for Borek's site, just put the "." inbetween chembuddy and com:
http://www.chembuddy.com/?left=pHcalculation&right=introductionacidbaseequilibrium#eq1.1
That'll explain K_{w}.

Given that [H+]=[C4H7O2] = 3.8 x 10^{3} M and [HC4H7O2] = 1 M, calculate K_{a} for this equilibrium.
How do you calculate K_{a}?
Borek's site said that for "HA <> H^{+} + A^{}," K_{a} = [H^{+}][A^{}] / [HA]
How do I get K_{a} from what's given in the question above?

Sorry for the typo in url, I'll correct it in just a moment.
You have everything given  you must identify now what is HA and A^{} in terms of the substance present in your solution. That's if you have already identified H^{+} ;)

H(C4H7O2) <> C4H7O2 + H+
Is it, "K_{a} = [H+][C4H7O2] / [H(C4H7O2)]."

OK, although formatting looks lousy ;)

So I would plug the numbers into that?
K_{a} = [3.8 x 10^{3} M][3.8 x 10^{3} M] / [1]
That doesn't seem right... Is that it?

Is that it?
Yes :)
(although I think the square brackets mean "conc^{n} of", so you can just stick with normal brackets ( & ) when doing the math!)

What is the pH of a solution with [H+] = 6 x 106?
Can someone just please tell me what the pH of this is???
I think its 5.22, but I'm wrong.

What is the pH of a solution with [H+] = 6 x 106?
Can someone just please tell me what the pH of this is???
I think its 5.22, but I'm wrong.
You are calculating it right, although you may be using too much significant figures in your answer.

Really? I told my teacher that I used a calculator, and did it the same way as they show in my book, but she said, "I assure you, it's incorrect."
Should I just say 5.2?

Really? I told my teacher that I used a calculator, and did it the same way as they show in my book, but she said, "I assure you, it's incorrect."
Should I just say 5.2?
Or even 5. As long as you have not missed something. What is exact wording of the question?

That is the exact wording.

Don't forget that pH is a measure of the acidity/basicity of a solution.
pKa/pKb is a measure of the acidity/basicity of a compound.

How would I find the pH of a 0.6M solution of NH3?
I am utterly perplexed by this question. :\

This is weak base.
http://www.chembuddy.com/?left=pHcalculation&right=pHweakacidbase

How would I find the pH of a 0.6M solution of NH3?
pH = 14  pOH
pOH =  lg [ OH^{} ]
NH_{3} (aq) + H_{2}O (l) <> NH_{4}^{+} (aq) + OH^{} (aq)
K_{b} = [ NH_{4}^{+} ] [ OH^{} ] / [ NH_{3} ]
let concentration of OH^{} be x (unit M)
K_{b} = x^{2}/(0.6  x)
This is a quadratic expression. Solve for x.
pOH =  lg x
pH = 14  pOH = 14 + lg x