Question

(You need to use Excel for this problem) For diprotic and trip rotic acid systems, the fractional composition equations were discussed in class. Use them to create a fractional composition diagram for arcenic acid (pK1 2.26, pK2-6.76 and pK 11.29). Calculate the fraction of each species at pH 5.0

Answer 1

Ans.

The fractional composition of a weak acid at any pH is given by: 10(pH-pKa) / [(1 + 10(pH-pKa)]

# Note that the fractional composition of a weak acid at its pKa is always 0.5, i.e. it's 50% ionized at its pKa value.

# Calculate the fractional composition of the three forms of arsenic acid using Excel and the above mentioned formula. Also include all the pKa values.

# Fractional composition of the three weak acid forms at pH 5 is shaded in dull yellow.

pH рка, 2.26 pka2 6.76 0.00 1.00 2.00 2.26 3.00 4.00 5.00 6.00 6.76 рказ 11.29 fАН, Тf AH? fА - 10он-pka) | 10он-pka) T 10(он-рка) 1+ 10он-pka13|1+ 10(он-ркаг|1+ 10рн-рказ) 0.0055 0.0000 0.0000 0.0521 0.0000 0.0000 0.3546 0.0000 0.0000 0.5000 0.0000 0.0000 0.8460 0.0002 0.0000 0.9821 0.0017 0.0000 0.9982 0.0171 0.0000 0.9998 0.1481 0.0000 1.0000 0.5000 0.0000 1.0000 0.6347 0.0001 1.0000 0.9456 0.0005 1.0000 0.9943 0.0051 1.0000 0.9994 0.0488 1.0000 0.9999 0.3390 1.0000 1.0000 0.5000 1.0000 1.0000 0.8368 1.0000 1.0000 0.9809 1.0000 11.0000 0.9981 7.00 8.00 9.00 10.00 11.00 11.29 12.00 13.00 14.00

Fractional Composition of Different forms of Arsenic acid 1.0000 0.8000 0.6000 fAH, PAH2- FAB- Fractional Composition 0.5000 0.4000 0.2000 0.0000 0.00 2.00 4.00 6.00 8.00 10.00 12.00 14.00 PH