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Offline Needaask

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titration question and pH
« on: April 23, 2013, 11:38:02 AM »
Hi, I had a lab session yesterday where we titrated an alkali into an acid. We were told to only rinse the receiving flask with distilled water as if we were to rinse it with the acid, there would be some acid left behind causing the number of moles of the acid to be greater. But I was thinking that if we rinse it with just water, won't the acid be diluted?

So even if the number of moles of acid remains the same, the pH would be closer to 7 after diluting the acid. So I'm thinking that I would need less alkali to bring the pH down, but still since the number of moles of acid remain the same, the same amount of alkali would neutralize it. But now that the pH of the acid is closer to 7 it suggests otherwise. So I'm having some trouble understanding these 2 seemingly opposing phenomenons..

Any help would be greatly appreciated :)

Offline Borek

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Re: titration question and pH
« Reply #1 on: April 23, 2013, 01:29:54 PM »
Dilution doesn't matter - it is stoichiometry that matters. Just by diluting you don't change the amount of acid, so the amount of base that has to be added doesn't change.

There is some fine print to dilution, but it requires delving quite deep.
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Offline Needaask

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Re: titration question and pH
« Reply #2 on: April 23, 2013, 07:42:57 PM »
Dilution doesn't matter - it is stoichiometry that matters. Just by diluting you don't change the amount of acid, so the amount of base that has to be added doesn't change.

There is some fine print to dilution, but it requires delving quite deep.

Oh so actually if we plot a graph of pH against volume of aalkali titrated, both would end at the same pH? Just that one would start with a higher pH that the other (cos of the dilution)?

Offline Borek

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Re: titration question and pH
« Reply #3 on: April 24, 2013, 03:10:29 AM »
Almost yes. There will be a minute change in the equivalence point pH, as it is a function of the titrated substance concentration. But in a typical situation (like diluting with 100 mL of water instead of 80 mL of water) change in equivalence point pH will be just a few hundreds of pH unit, while adding a single drop of titrant changes pH by several tenths of pH units (if not more). This means for all practical purposes small mistakes in dilution don't matter.
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Offline Needaask

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Re: titration question and pH
« Reply #4 on: April 24, 2013, 04:39:05 AM »
Almost yes. There will be a minute change in the equivalence point pH, as it is a function of the titrated substance concentration. But in a typical situation (like diluting with 100 mL of water instead of 80 mL of water) change in equivalence point pH will be just a few hundreds of pH unit, while adding a single drop of titrant changes pH by several tenths of pH units (if not more). This means for all practical purposes small mistakes in dilution don't matter.

Oh! But I still can't seem to relate the two together. Even if the acid solution gets diluted and has a higher pH, it still requires the same number of moles of the alkali to completely neutralize it. So how does pH play a role here?

I'm thinking that usually if we say something has a higher pH, it would require less alkali to neutralize it compared to something else with a lower pH. I think that's why I'm feeling puzzled about this case here.

Thanks for the help Borek :)

Offline Borek

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Re: titration question and pH
« Reply #5 on: April 24, 2013, 05:10:24 AM »
How much NaOH needed to neutralize 1 mL of 1M HCl solution? What is initial pH of the acid solution?

How much NaOH needed to neutralize 1000 mL of 0.001M HCl solution? What is initial pH of the acid solution?

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Offline Needaask

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Re: titration question and pH
« Reply #6 on: April 25, 2013, 04:06:39 AM »
How much NaOH needed to neutralize 1 mL of 1M HCl solution? What is initial pH of the acid solution?

How much NaOH needed to neutralize 1000 mL of 0.001M HCl solution? What is initial pH of the acid solution?

They would require both 0.001 moles of sodium hydroxide for complete neutralization. However the pH of the first acid is 0 while the pH of the second one is 3.

So this would mean that both solutions would require the same number of sodium hydroxide. So actually is the pH useless here?

Offline Borek

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Re: titration question and pH
« Reply #7 on: April 25, 2013, 05:53:19 AM »
So actually is the pH useless here?

Yes. It doesn't have to work even for solutions of the same volume.
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Offline Needaask

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Re: titration question and pH
« Reply #8 on: April 26, 2013, 01:59:45 AM »
So actually is the pH useless here?

Yes. It doesn't have to work even for solutions of the same volume.

Oh! Then actually why would we need to learn about pH since its useless in so many cases?

Offline Borek

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Re: titration question and pH
« Reply #9 on: April 26, 2013, 02:46:49 AM »
pH doesn't tell anything quantitative about the solution composition, but it tells a lot about how the solution (or more precisely compounds in the solution) behave. It is one of the most important factors describing solution properties.
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