Normality, equivalents, and titration

[totalcommand]

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Hi, this normality concept is really frustrating me for some reason, and examkrackers just passes over it. How is it used in titrations? How do I convert from normality to molarity? Why is it useful?

Thanks so much.

 

[TIGIBedHead]

Normality has to do with equivalents of acid.

For example, when HCl dissociates, there is one H+ ion per moles of acid. Since it has one equivalent, molarity is equal to normality.

H3PO4, on the other hand, produces 3 H+ ions per mole of acid, so its normality would be three times its molarity.

 

[totalcommand]

Ok, that makes sense. What kind of questions might be asked about it?

So if they say given a 1N H3PO4 solution, what is the [H+], the answer would be 1M, but the [PO4] would be .33M?

Thanks.

 

[TIGIBedHead]

Close, but flip those values around. Since we're looking at equivalents of acid, you would take the [H+] and multiply it by 3 to get Normality. So 3 x M = N.

The acid produces one equivalent of PO4, so M=N in this case (like in the HCl example, where there is one equivalent of Cl per mole of acid).

 

[totalcommand]

Close, but flip those values around. Since we're looking at equivalents of acid, you would take the [H+] and multiply it by 3 to get Normality. So 3 x M = N.

The acid produces one equivalent of PO4, so M=N in this case (like in the HCl example, where there is one equivalent of Cl per mole of acid).

Hmm, I'm still a little confused. I don't really understand how you get a higher molarity of PO4 than H when there are 1 PO4: 3H

Normality = (# of equivalent H) X molarity right?

So if it is a 1 Normal solution, doesn't it mean 1N = 3 X molarity, so molarity of H3PO4 would be .33M?

H3PO4 ----> 3H + 1 PO4
If you set up a reaction equation where you get 3H+ dissociating fully, wouldn't you get [H+] = 3*0.33Molar = 1Molar? And [PO4] = .33Molar?

 

[topdogg82]

that seems right to me- a 1 N H3PO4 solution = .33 M H3PO4 soln. so the PO4 conc. should be equivalent to the initial H3PO4 conc. based on stoichiometry, while H+ conc. should be 3x, or 1 M.

 

[TIGIBedHead]

Edited: Okay, apparently it doesn't work like that. Normality applies to the reactive species. So if we have a 1M solution of H3PO4, the Normality is 3 x M = 3 N.

 

[hippocampus]

what is the point of titration? why do people do it?

 

[TIGIBedHead]

what is the point of titration? why do people do it?

You react an x mL of acid and y mL of base. If you know the concentration of one (the standard), you can find the unknown concentration of the other.

 

[totalcommand]

what is the point of titration? why do people do it?

You can figure out the concentration of an unknown using titration.
In acid-base titration, you can figure out the concentration of weak acid by adding a known amount of known concentration NaOH:

(Normality of the titrant)(Volume of titrant required to reach equivalence point) = (Normality of unknown) (Volume of unknown)

Then from the concentration of the weak acid, you can figure out the pKa of the weak acid.

THanks for the help guys.

 

[hippocampus]

why would the person want to know the concentration of the solution? are they going to use it later for something else? if theyre going to use it later, then they are ruining the solution by adding a titrant. that means...they save some of the solution so that they can use later?

also, how do they choose the indicator? how do they know which indicator to use? how do they know that at that certain pH, the equivalence point will occur?

thanks

 

[totalcommand]

Edited: Okay, apparently it doesn't work like that. Normality applies to the reactive species. So if we have a 1M solution of H3PO4, the Normality is 3 x M = 3 N.

Hmm, but isn't this still consistent with saying a 1N H3PO4 solution would be .33M H3PO4 (which must mean [H] = 1M)?

We can just look at it the other way around. If we have 1M [H+] and it reassociates with PO4, we'll get 0.33M H3PO4. Then, Normality is 3 x M = 3 x 0.33 = 1N H3PO4.

 

[totalcommand]

why would the person want to know the concentration of the solution? are they going to use it later for something else? if theyre going to use it later, then they are ruining the solution by adding a titrant. that means...they save some of the solution so that they can use later?

Yep, they can save some of the solution.

You can figure out the molar mass of the compound - you learn the number of moles from the titration, and you might know the mass you added to the solution.

You can also figure out the pKa from the half-equivalence point.

also, how do they choose the indicator? how do they know which indicator to use? how do they know that at that certain pH, the equivalence point will occur?

thanks

All indicators have a certain range of about +/- 1 above and below the indicator's pKa (indicators are buffers themselves, except they change color when they are neutralized).

If the pKa of the unknown concentration is known prior to the titration, you can get a good estimate of the equivalence point from calculations. Then, you choose an indicator whose pKa is +/- 1 within the equivalence point. What you will measure to determine the concentration of the unknown is the amount of NaOH necessary to cause a color change in the indicator.

If the pKa of the unknown concentration is not known, I think you'll just have to guess. If the unknown is a weak acid, the equivalence point will be above pH7.