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Topic: pH value after dilution  (Read 2320 times)

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Offline Lourdes

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pH value after dilution
« on: June 03, 2017, 12:49:39 PM »
Hi!

Now , I know that most of you are going to say that nicotinic acid is a strong acid, but our professor wants us to use the formula for weak acids instead, who knows why :)

Can somebody check if they get the same answers as I do?

300 g of nicotinic acid (molar mass: 123 g / mol, pKa value: 2.03) are dissolved in 250 ml of water.
A) What is the pH value of the solution?
B) What is the pH value of the solution after six-fold dilution?

I first calculated the amount of substance, I got 2,4 moles.
Then I calculated the concentration.
I got about 9.6 mol /l. And with the pH-value formula for weak acids (pKs-value is small, but according to our professor we should see this as a weak acid!) I got a pH value of 0.52.

For B) I used: c1 * V1 = c2 * V2 considering that V2 = V1 * 6

The concentration for the new solution would thus be 1.6 mol / l. My pH value after my six-fold solution would therefore be 0,91.

Thank you very much guys for helping me!

Offline Borek

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Re: pH value after dilution
« Reply #1 on: June 03, 2017, 03:33:32 PM »
I got about 9.6 mol /l. And with the pH-value formula for weak acids (pKs-value is small, but according to our professor we should see this as a weak acid!) I got a pH value of 0.52.

Have you tried to calculate pH ignoring the pKa? Comparing results will tell you why you can't treat it as a strong acid.

0.53 and 0.93 look about right (even if they don't make sense for so highly concentrated solutions).
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Offline Lourdes

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Re: pH value after dilution
« Reply #2 on: June 03, 2017, 03:43:28 PM »
Thank you very much! Do you think that the way I calculated it with c1 * V1 = c2 * V2 is correct?

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