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Author Topic: Acid pH value of a Na2SO3 + H2O solution  (Read 8401 times)

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giofrida

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Acid pH value of a Na2SO3 + H2O solution
« on: May 31, 2011, 10:17:38 PM »

I'm so frustrated, I can't understand that.

A few days ago we did an experiment: saline hydrolysis

we put some (a teaspoon) Na2SO3 in 10 cm3 of distilled water and then we took the pH, which was approximately 5.

I know that Na+ should not interfere in water hydrolysis, and that SO3-2 act as a weak base.
Then the pH value should be greater than 7, am I right?

Can someone explain me why it happened? :'(
I'm sure not to have contaminated the solution.
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AWK

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Re: Acid pH value of a Na2SO3 + H2O solution
« Reply #1 on: May 31, 2011, 10:35:17 PM »

Na2SO3 should be slihtly alkaline
 This was probably Na2S2O5 solution which in water forms NaHSO3
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giofrida

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Re: Acid pH value of a Na2SO3 + H2O solution
« Reply #2 on: May 31, 2011, 10:46:59 PM »

but I'm sure it's not.. Better: my professor is sure.
He only gave us the compounds and we did the work. He was also agreeing with our results..

Only one question: there's no chance it was Na2SO3, having an acid pH value?
Maybe it could be caused by an excess of H3O+ ions in the reaction:

Na2SO3 + H2O  :rarrow: 2NaOH + H2SO3
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Nobby

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Re: Acid pH value of a Na2SO3 + H2O solution
« Reply #3 on: May 31, 2011, 11:12:45 PM »

Sodiumsulfite has an pH of 8.8

acidic is Sodiumbisulfite  NaHSO3 pH 5
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giofrida

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Re: Acid pH value of a Na2SO3 + H2O solution
« Reply #4 on: May 31, 2011, 11:48:47 PM »

I know that!  :P but I copied that formula directly from the blackboard, and my schoolmates too.
At this point the only way to get a pH = 5, when you should not, is to work with a dirty beacker. (Maybe other students didn't wash the beacker the hour before, and put it on the shelf as new ;D it often happens)

thank you for reply anyway.
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AWK

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Re: Acid pH value of a Na2SO3 + H2O solution
« Reply #5 on: May 31, 2011, 11:57:35 PM »

but I'm sure it's not.. Better: my professor is sure.
He only gave us the compounds and we did the work. He was also agreeing with our results..

Only one question: there's no chance it was Na2SO3, having an acid pH value?
Maybe it could be caused by an excess of H3O+ ions in the reaction:

Na2SO3 + H2O  :rarrow: 2NaOH + H2SO3
This reaction gives an excess of OH-
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giofrida

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Re: Acid pH value of a Na2SO3 + H2O solution
« Reply #6 on: June 01, 2011, 12:04:42 AM »

This reaction gives an excess of OH-

 ;Di tried..
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Bert95

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Re: Acid pH value of a Na2SO3 + H2O solution
« Reply #7 on: June 02, 2011, 01:26:30 PM »

Did you check the pH of the water before the salt was added? How was the pH measured? Sources of acidity could be impurities of the salt or your labware (beaker, spoon,...)
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