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Chemistry Forums for Students => High School Chemistry Forum => Topic started by: trinhn812 on February 21, 2008, 08:44:21 PM

Title: Challenge problem: Stroiciometric points (titration) and pH, Ka caculations
Post by: trinhn812 on February 21, 2008, 08:44:21 PM
malonic acid is a diprotic acid. in the titration of malonic acid with NaOH, stoichiometric points occur at pH 3.9 and 8.8
A 25.00 ml sample of malonic acid of unknown concentration is titrated with .0984 M NaOH requiring 31.50 mL of the NaOH soultion to reach the phenophthalein end point. calculate the concentration of malonic acid in the unknown solution.

At pH =8.8
H+ = 1.65e-9 = let's label this as x
Okay so I think the equation is
  A- + H20 = HA + OH-

A- is  y/(31.50mL+31.50mL) = z

Ka=(x)(x)/ z  

So the problem I'm having is, what is Ka? And why are there two stoichiometric points? Do I just pick one for x?
Title: Re: Challenge problem: Stroiciometric points (titration) and pH, Ka caculations
Post by: Borek on February 22, 2008, 03:58:56 AM
You start with H2A - try to imagine how it is neutralized. Will HA- neutralization start before H2A is neutralized?