* Determining the equivalent mass and dissociation constant of an unknown weak a
ID: 1014439 • Letter: #
Question
* Determining the equivalent mass and dissociation constant of an unknown weak acid by titrimetry
suppose that a student doing this experment failed to dry the KHP before using it to standarize the NaOH solution
1) would her calculated molarity of the NaOH solution probably be too high or too low? briefly explain
2) How would the error you described in (1) affect the calculated equivalent mass of the unknown acid? briefly explain
3) How would the error you described in (1) affect the calculated Ka of the unknown acid? briefly explain
Suppose that a stuent peforming this experiment mistakenly calibrated the PH meter using PH6 buffer, instead of PH7 buffer. as a result, all PH meter readings were too low.
1) Would this procedural error have affected the calculated equivalent mass of the unknown aid? briefly explain
2) Would this procedural error have affected the experimentally determined PKa of the unknown acid? briefly explain
Explanation / Answer
1. Since the student was using wet KHP to standarize NaOH; hence mass of KHP (actual ) will be less than calculated value. For this reson NaOH will more amount and calculated Molarity will be high than expected.
2. Since in titrimetry we use the formula
N1V1=N2V2
and N (mormality) of the NaOH solution is high as we have seen in step one, so the normality of unknown acid will also be incorrect.
weight of acid = equivalent mass x normality x volume in litre
3. Ka = [H+][A-] /[HA]
where [HA] is the concentration of the weak acid and hence the dissociation constant of acid will be incorrect.
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