Gen Chem 2 ICE Table & Equilibrium Equation

Question

Determine the concentration of C6H5NH3+ in a buffer solution by constructing an ICE table, writing the equilibrium constant expression, and use this information to determine the concentration of C6H5NH3+. The Kb for C6H5NH2 is 3.8 x 10^-10.

A buffer solution contains dissolved C6h5NH2 and C6H5NH3Cl. The initial concentration of C6H5NH2 is 0.50 M. The pH at equilibrium of the buffer is 4.20. Let x represent the original concentration of C6H5NH3+ in the water. Fill in the ICE table with the appropriate value for each involved species to determine concentrations of all reactants and products.

Answer 1

C₆H₅NH₂ + HCl ===> C₆H₅NH₃Cl

0.50....................................0.................Initial

-x.......................................+x................Change

0.50-x..................................x................Equilibrium

pH = pKa + log [C6H5NH2] / [C6H5NH3Cl]

pH = 4.20

pKa = -log Ka and Ka = 1x10^-14/3.8x10^-10 = 2.63x10^-5 and pKa = 4.58

4.20 = 4.58 + log (0.5-x/x)

log (0.5-x/x) = -0.38

0.5-x/x = 0.417

x = 0.353 M C6H5NH3Cl

0.50-x = 0.147 M C6H5NH2