This is a graph of a reaction with a weak acid and strong base. It is showing th

Question

This is a graph of a reaction with a weak acid and strong base. It is showing the titration of 40.00 mL of.1000M propanoic acid (HPr) with .1000M NaOH. Titration of propanoic acid with NaOH 14 10 6 10 20 30 40 50 60 Volume (mL) a. How many mL of acid are needed to reach equivalence point? b. Calculate the pH after addition of 15.00 mL of base? c. What is the pH after addition of 20.0 mL of base? What is special about this point? Label it on your graph. d. Calculate the pH after addition of 30.00 mL of base. e. Calculate the pH at equivalence point

Explanation / Answer

a)

millimoles of propanoic acid = 40 x 0.1 = 4

at equivalence point :

millimoles of acid = millimoles of base

4 = 0.1 x V

volume of NaOH at equivalence point = 40 mL

b)

millimoles of base = 15 x 0.1 = 1.5

acid + NaOH ------------> salt + water

4 1.5 0 0

2.5 0 1.5   

pH = pKa + log [1.5 / 2.5]

= 4.886 + log [1.5/2.5]

= 4.66

pH = 4.66

c)

millimoles of base = 20 x 0.1 = 2

this is half equivalence point : so here

pH = pKa

pH = 4.89

d)

millimoles of base = 30 x 0.1 = 3

pH = 4.886 + log [3/1]

= 5.36

e)

here only salt remains .

salt concentration = 4 / 40 + 40 = 0.05

pH = 7 + 1/2 (pKa + log C)

= 7 + 1/2 (4.886 + log 0.05)

= 8.79

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