Chemical Forums
Chemistry Forums for Students => High School Chemistry Forum => Topic started by: Shea on August 22, 2006, 06:50:27 PM
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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?
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Never mind, I got it now. I'm even more lost though on this.
Write an expression for Kp in the following:
2NO2 -> N2O4
and
Write an expression for the ionic product of water Kw
and
Write an expression for Kc for the dissociation reaction of butanoic acid
H(C4H7O2) <-> C4H7O2- + H+
I don't get the Kp, Kc, and Kw. The book doesn't explain this, even though it says on the cover that it makes chemistry easy to learn with full explanations...
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And what Exactly does log mean in this?
http://en.wikipedia.org/wiki/Logarithm
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I don't get the Kp, Kc, and Kw
http://en.wikipedia.org/wiki/Reaction_quotient
Kp uses pressures, Kc and Kw use concentrations. Kw is just Kc for water dissociation, although it is usually assumed that water concentration is constant.
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Thanks for explaining that, but what do you think the questions mean by, "write an expression?"
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Look at this page:
http://www.chembuddy.com/?left=pH-calculation&right=introduction-acid-base-equilibrium#eq1.1
Equation 1.1 is the expression for the acid dissociation constant.
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Um, that page wont open. When I click on it, it goes to a site that offers some highly questionable search topics...
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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=b93c05b90d45a665308d61570d8f03d3cb77b1bb)(https://www.chemicalforums.com/proxy.php?request=http%3A%2F%2Fupload.wikimedia.org%2Fwikipedia%2Fcommons%2F5%2F5c%2FGleichgewicht.png&hash=1f54a61616564155b10d011d435ea689afc5543c)(https://www.chemicalforums.com/proxy.php?request=http%3A%2F%2Fupload.wikimedia.org%2Fmath%2F9%2F2%2Fd%2F92d799525c5bec2655bd431487cd27ac.png&hash=eb657b8dbf18db5d4a38e2817ce5254f2a09885c)
and
(https://www.chemicalforums.com/proxy.php?request=http%3A%2F%2Fupload.wikimedia.org%2Fmath%2F8%2F2%2Fe%2F82e273f9e108ceaea60dd7f69a4ab244.png&hash=3cf99ec292249714b5eeee5bbac1acb81b502c8e)
When they tell you to write an expression they want you to fill in the above expression (ignore kf and kb).
For Kp 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=pH-calculation&right=introduction-acid-base-equilibrium#eq1.1
That'll explain Kw.
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Given that [H+]=[C4H7O2-] = 3.8 x 10-3 M and [HC4H7O2] = 1 M, calculate Ka for this equilibrium.
How do you calculate Ka?
Borek's site said that for "HA <-> H+ + A-," Ka = [H+][A-] / [HA]
How do I get Ka from what's given in the question above?
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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+ ;)
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H(C4H7O2) <-> C4H7O2- + H+
Is it, "Ka = [H+][C4H7O2-] / [H(C4H7O2)]."
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OK, although formatting looks lousy ;)
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So I would plug the numbers into that?
Ka = [3.8 x 10-3 M][3.8 x 10-3 M] / [1]
That doesn't seem right... Is that it?
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Is that it?
Yes :)
(although I think the square brackets mean "concn of", so you can just stick with normal brackets- ( & ) when doing the math!)
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What is the pH of a solution with [H+] = 6 x 10-6?
Can someone just please tell me what the pH of this is???
I think its 5.22, but I'm wrong.
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What is the pH of a solution with [H+] = 6 x 10-6?
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.
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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?
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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?
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That is the exact wording.
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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.
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How would I find the pH of a 0.6M solution of NH3?
I am utterly perplexed by this question. :-\
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This is weak base.
http://www.chembuddy.com/?left=pH-calculation&right=pH-weak-acid-base
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How would I find the pH of a 0.6M solution of NH3?
pH = 14 - pOH
pOH = - lg [ OH- ]
NH3 (aq) + H2O (l) <-> NH4+ (aq) + OH- (aq)
Kb = [ NH4+ ] [ OH- ] / [ NH3 ]
let concentration of OH- be x (unit M)
Kb = x2/(0.6 - x)
This is a quadratic expression. Solve for x.
pOH = - lg x
pH = 14 - pOH = 14 + lg x