Ammonia (NH 3 ) is a weak base with a pKb of 4.75. Its conjugate acid, ammonium

Question

Ammonia (NH3) is a weak base with a pKb of 4.75. Its conjugate acid, ammonium (NH4+) has a pKa of 9.25. Set up a titration using 20.00 mL of HCL to titrate 20.00 mL of aqueous ammonia solution.

Concentration of HCl soln: 0.1044 mol/L

Initial buret reading (mL): 0.00

Buret reading at equivalence pt (mL): 14.41

Calculate the following:

a.) Moles of HCl added at equivalence pt

b.) Moles of NH3 in sample

c.) NH3 concentration (mol/L)

d.) pH at equivalence point

e.) pH at half the volume required to reach equivalnce

f.) pKa of ammonium (NH4+)

g.) pKb of ammonia

Thank you!

Explanation / Answer

a.) Moles of HCl added at equivalence pt

   = 0.1044*14.41/1000 = 0.0015 mol

b.) Moles of NH3 in sample

   at equivalence pt

No of mols of NH3 = no of mol of HcL

No of mols of NH3 IN SAMPLE = 0.0015 mol

c.) NH3 concentration (mol/L)

   = 0.0015/20*1000 = 0.075 mol/L

d.) pH at equivalence point

pH = 7-1/2(pkb+logC)

c= concentration of salt formed = 0.0015/34.41*1000 = 0.0436 M

   = 7 - 1/2(4.75+log0.0436)

    = 5.3

e.) pH at half the volume required to reach equivalnce

pH at half equivalence point = 14- pkb = 14 - 4.75 = 9.25

f.) pKa of ammonium (NH4+)

pka = 14 - pkb = 14 - 4.75 = 9.25

g.) pKb of ammonia

pkb = 4.75

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