Chemistry ICE Table

Question

Determine the number of grams of NH₄Br needed to prepare an aqueous solution at a specific pH by constructing an ICE table, writing the equilibrium constant expression, and using this information to determine the initial grams of NH₄Br required. The Kb of NH. is 1.8 x 10^-5.

An unknown quantity of NH₄Br is dissolved in 1.00 L of water to produce a solution with a pH = 5.16. Let x represent the original concentration of NH₄⁺ 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

Looking at the hydrolysis of NH4Br (and ignoring Br- as a spectator ion), we have...

NH₄⁺ + H2O ==> H3O⁺ + NH₃

If the pH = 5.16, this means that [H3O+] = 1x10^-5.16 = 6.92x10^-6 M

NH₄⁺ + H2O ==> H3O⁺ + NH₃

x...........................0.............0...........Initial

-6.92x10^-6.....+6.92x10^-6....+6.92x10^-6...Change

x-6.92x10^-6.....6.92x10^-6.....6.92x10^-6...Equilibrium

Ka for NH4 = 1x10^-14 / Kb = 1x10^-14/1,8x10^-5 = 5.56x10^-10

Ka = 5.56x10^-10 = [H3O+][NH3] / [NH4⁺] = (6.92x10^-6)² / x-6.92x10^-6

Simplify to

Ka = 5.56x10^-10 = (6.92x10^-6)² / x

x = 0.0861 M NH4Br

molar mass NH4Br = 97.9 g / mol

Grams NH4Br used = 0.0861 mols x 97.9 g / mol = 8.43 g NH4Br

(be sure to check all of the math)