Chemical Forums
Chemistry Forums for Students => Physical Chemistry Forum => Topic started by: neon on April 16, 2014, 01:56:15 PM
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A 2.5 gm impure sample containing weak monoacidic base (Mol wt =45) is dissolved in 100 ml water and titrated with 0.5M HCl when 1/5 of the base was neutralised the pH was found to be 9 and at equivalent point pH of solution is 4.5. Find concentration of salt at equivalence point.
Let the initial concentration of base be c. When 1/5 of the base is neutralised amount of base left is equal to 4c/5 and concentration of salt formed is c/5.
Using Henderson's equation
pOH = pKb +log(1/4)
This gives me pKb = 5.6.
At equivalence point
Total amount of base is converted into salt. Thus concentration of salt is c.
Assuming hydrolysis of the salt
pH = 7-1/2pKb-log(c)
This gives me C=0.5. But the correct answer says that it should be 0.25.
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pH = 7-1/2pKb-log(c)
Check it out.
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What's wrong with that equation? In salt hydrolysis of a salt containing weak base and strong acid,
[itex][H+] = \sqrt{\dfrac{K_w C}{K_b}} [/itex]
Converting the above into logarithms gives exactly the same expression I wrote earlier.
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Converting the above into logarithms gives exactly the same expression I wrote earlier.
That's where you are wrong. Check your math.
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Oops! That was a silly mistake. :P Thanks for pointing out.