Chemical Forums

Chemistry Forums for Students => High School Chemistry Forum => Topic started by: alina0031 on May 17, 2013, 12:49:36 AM

Title: pH of NaHSO4
Post by: alina0031 on May 17, 2013, 12:49:36 AM
To a chemistry experiment there is made an aqueous dissolution of NaHSO4. There is weighed 0,82 gram of the salt.  The salt is poured in a graduated flask where there is 100 mL of water .

1) What is the dissolution pH?

2) The is taken 10 mL of the dissolution with a pipette. The 10 mL of the dissolution is poured in a new graduated flask with 100 mL of water. What is the pH of the diluted dissolution?

(Sorry about my English i'm from Germany)
Title: Re: pH of NaHSO4
Post by: Hunter2 on May 17, 2013, 01:34:56 AM
What is your own attempt?  You need to calculate the moles of the solution. You need the pKA of hydrogensulfate and  the calculation formulas for weak acids.
Title: Re: pH of NaHSO4
Post by: alina0031 on May 17, 2013, 02:21:58 AM
Quote from: Hunter2 on May 17, 2013, 01:34:56 AM
What is your own attempt?  You need to calculate the moles of the solution. You need the pKA of hydrogensulfate and  the calculation formulas for weak acids.

I really don't know what to do.
I know that the molar mass of NAHSO4 is 120,0599 g/mol.
so the n = m / M => n = 0,82 g / 120,0599 =  0,00682992406291 mol

Then I say :
c = n / V => c = 0,00682992406291 mol / 0,1 L = 0,0682992406291 mol/L

Then I don't know what to do. And I'm not sure about the last part.


Title: Re: pH of NaHSO4
Post by: Hunter2 on May 17, 2013, 02:48:56 AM
pKA from HSO4- is 1,9

http://www.msina.de/Schule/Chemie/Glossar/pKS-Werte/pks-werte.html

pH = ½(pKa - log [HA]0)

http://www.youtube.com/watch?v=A98e4VlcJFk
Title: Re: pH of NaHSO4
Post by: Borek on May 17, 2013, 03:02:36 AM
Quote from: Hunter2 on May 17, 2013, 02:48:56 AM
pH = ½(pKa - log [HA]0)

While your general advice about using Ka and weak acid formulas is OK, this is wrong.

Alina: you have a 0.0683M solution of a weak acid with Ka=10-1.9. Does it ring a bell?
Title: Re: pH of NaHSO4
Post by: alina0031 on May 17, 2013, 05:46:01 AM
Quote from: Borek on May 17, 2013, 03:02:36 AM
Quote from: Hunter2 on May 17, 2013, 02:48:56 AM
pH = ½(pKa - log [HA]0)

While your general advice about using Ka and weak acid formulas is OK, this is wrong.

Alina: you have a 0.0683M solution of a weak acid with Ka=10-1.9. Does it ring a bell?


But are my calculations correct?
Title: Re: pH of NaHSO4
Post by: alina0031 on May 17, 2013, 05:51:21 AM
- And is this correct:
pH = 1/2 *(pKs - logcs) => 0,5*(1,99-log(0,0683)) = 1,577
Title: Re: pH of NaHSO4
Post by: Borek on May 17, 2013, 06:18:10 AM
You were OK up to the concentration of the hydrogensulfate. Unfortunately, the simplified formula Hunter suggested doesn't work here, it works only if the dissociation fraction of the weak acid is below 5%, which is not the case.

You should use ICE table.
Title: Re: pH of NaHSO4
Post by: alina0031 on May 17, 2013, 06:21:26 AM
Quote from: Borek on May 17, 2013, 06:18:10 AM
You were OK up to the concentration of the hydrogensulfate. Unfortunately, the simplified formula Hunter suggested doesn't work here, it works only if the dissociation fraction of the weak acid is below 5%, which is not the case.

You should use ICE table.

Thank you for helping me.
I am really lost. Can you write the solution.
Title: Re: pH of NaHSO4
Post by: alina0031 on May 17, 2013, 06:39:24 AM
Ups, I just found out how to solve it
Title: Re: pH of NaHSO4
Post by: Hunter2 on May 17, 2013, 07:46:58 AM
Then light the bulb please. Because I also get lost.

Erleuchte uns mal.
Title: Re: pH of NaHSO4
Post by: Borek on May 17, 2013, 08:10:56 AM
http://www.chembuddy.com/?left=pH-calculation&right=pH-weak-acid-base