Question

please help me do the graph and find the balie of pka . i know pka=_log ka . but how to find ka throught the graph . you could answer the questions based on the data . please

pKal pKal al al al Average Ka Standard deviation pKa2 Ka2 a2 pK,2 a2 Standard deviation Average Ka2

Answer 1

First plot pH of analyte vs volume of NaOH (mL) taking pH along X axis (abscissa) and volume of NaOH along Y axis (ordinate) putting the values from 1st observation data. You'll get an almost S shaped curve. Next plot the graph of 1st derivative of pH with respect to volume of NaOH i.e.,

delta pH/ delta V = [(pH₃ +pH₂)/2 - (pH₂+pH₁)/2] / [(V₃+V₂)/2 - (V₂+V₁)/2] ;

pH₁, pH₂, pH₃ .... are consecutive pH values and V₁,V₂,V₃ ..... are consecutive volume of NaOH.

the highest peak of the curve will correspond to the equivalence point of the titration, Identifying the point, determine its corresponding volume value by drawing a perpendicular to the Y axis (delta pH/delta V) from the point, Now going back to the 1st plot, assign the corresponding pH for that volume of NaOH.

pK_a is the pH value corresponding to half the volume at equivalence point. For example, if the volume of NaOH at equivalence point is 20 mL, pK_a value will be just equal to the pH at volume of 10 mL of NaOH. And from pK_a, K_a value can be calculated by pK_a= - logK_a.

Similarly you can have the values for 2nd and 3rd observations respectively.