Chemical Forums
Chemistry Forums for Students => High School Chemistry Forum => Topic started by: peterpan1372 on September 14, 2017, 03:58:15 PM
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Hello,
I have a question regarding the picture...
I know the formula: V1*M1*n2=V2*M2*n1
my only question is: how can I know n1 and n2?
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What are n1 and n2?
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n1 and n2 are the number of molecules...
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Number of molecules in what?
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n1=NaOH
n2=diprotic acid...
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Assume any diprotic acid, can you predict with how many molecules of NaOH will one molecule of such acid react?
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well, diprotic (as the name says) has 2protons. since NaOH has only one H molecule, there must be two NaOH, or?
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well, diprotic (as the name says) has 2protons
Yes.
since NaOH has only one H molecule
No, NaOH doesn't have one H molecule. If anything, it has one H atom per NaOH molecule. Which is not that important, as H from the NaOH doesn't take part in neutralization.
Can you write reaction equation of a neutralization reaction between - say - NaOH and HCl?
there must be two NaOH, or?
Miraculously, that's correct.
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NaOH + HCl -> NaCl + H2O
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NaOH + HCl -> NaCl + H2O
So it is not H of NaOH, but some other part of the molecule that reacts during the neutralization, isn't it?
As you got correct values of n1 and n2 question is solved.
In general whenever dealing with titration it is always about reaction equation and stoichiometry:
http://www.titrations.info/titration-calculation
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NaOH + HCl -> NaCl + H2O
Good start. Now, ignore your formula, V1*M1*n2=V2*M2*n1, please. Although you may like it, and may have been taught it, you don't seem to really understand it.
Now, working with the actual problem:
What volume of 0.10 M NaOH is needed to neutralize 30 mL of 0.20 M of the diprotic acid H2(random anionate).
Look and see what I've done:
1). First of all, I wrote it down, instead of taking a picture with my phone, this gives a person a chance to study a problem, to be sure they understand it.
2). I changed the units. If you don't like that or don't understand that, no problem. You can use your oid units, if you want to keep writing them.
3). They say diprotic acid, but they don't say which one. No matter, I made up a diprotic acid. I could have called it H2(peterpanate). Notice though: they said diprotic, and you used a monoprotic acid as an example. Sorry, but your phone wasn't able to help you, whereas writing it down would. Please don't use HCl as an example again, for this problem, it just means that you won't read our help, and just want the answer given to you.
OK, lets try changing units again, to see if you have something to visualize:
You have a two stacks of 30 copper coins. You have to make stacks of coins, one silver coin, with one copper coin. How many silver coins do you need?
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60?
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they said diprotic, and you used a monoprotic acid as an example
I asked to use HCl as an example of neutralization, not as a part of solving the problem, so don't blame OP :)
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they said diprotic, and you used a monoprotic acid as an example
I asked to use HCl as an example of neutralization, not as a part of solving the problem, so don't blame OP :)
I still dont know why there are two molecules NaOH and one H2...
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I still dont know why there are two molecules NaOH and one H2...
There is no H2 there.
Do you know what acids are? What bases are? How do they react? Sorry, we are not going to teach you everything from scratch and it looks like your problems stem from the fact you have not covered the most basic subjects. You need to go back and work from the beginning.
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I thought that since H2 are prton donators they donate their protons to NaOH, giving them two molecules... right?
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If by H2 you mean two acidic protons in the diprotic acid, you are to some extent right, but you misuse the nomenclature in a way that makes it almost impossible to follow your posts. Nothing is "giving two molecules", I have no idea what you even mean by that.
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H2 donates its protons.. thats what I meant...
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H2 (or rather H2) is a molecule of hydrogen and it doesn't donate protons.
Proton is not a molecule.
As I said - you are misusing words and symbols making it very difficult to follow what you mean.
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could you tell me then why NaOH has two molecules please?
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could you tell me then why NaOH has two molecules please?
NaOH doesn't have two molecules, I have no idea what you are asking about.
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well, why is n1=2 and n2=1 (2NaOH and one H2)...
the answer to the question is 120ml ( as the solutions states)
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It is all in the stoichiometry of the neutralization of the diprotic acid.
What diprotic acids do you know?
How does an acid react with a base?
You have already wrote reaction equation of a reaction between HCl and NaOH. Can you write similar reaction equation for a diprotic acid, any of those you know?
And stop writing H2 when you mean diprotic acid, I told you several times it is wrong and confusing. Common way (and one that will be almost always correctly understood) of denoting diprotic acids is H2A.
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2NaOH + H2SO4 = Na2SO4 + 2H2O...
The way I understood it is that there will be two NaOH because of the fact that you also have to equate the reaction.. right?
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Yes. Or, to put it slightly different - NaOH contains a single OH- anion, capable of neutralizing a single acidic proton. Diprotic acid has two such protons, so you need two OH- anions (or two NaOH molecules) to neutralize one molecule of the acid. <- pay attention to the nomenclature and the way it is used.