IV.15. CALCULATIONS INVOLVING Kb

IV.15. CALCULATIONS INVOLVING Kb

(Hebden p. 152 – 154)(Hebden p. 152 – 154)

•Need to calculate Kb from Ka table.

•Basic solutions, so use [OH-], not [H3O+]

•Need to calculate Kb from Ka table.

•Basic solutions, so use [OH-], not [H3O+]

p.153: #84p.153: #84SO3

-2 + H2O HSO3- + OH-

I #M - 0 0

C - x - +x +x

E (#M - x) - x x

SO3-2 + H2O HSO3

- + OH-

I #M - 0 0

C - x - +x +x

E (#M - x) - x x

pH = 9.69

pOH = 4.31

[OH-] = 4.8978 x 10-5 M

So x = 4.90 x 10-5 M

pH = 9.69

pOH = 4.31

[OH-] = 4.8978 x 10-5 M

So x = 4.90 x 10-5 M

Kb = Kw/Ka = 1.0 x 10-14 / 1.0 x 10-7

= 1.0 x 10 -7

Kb = [HSO3-][OH-]/ [SO3

-2]

= (1.0 x 10 -7)2 / (#M - 4.90 x 10-5)

*Assume x (4.90 x 10-5) is so small so…

1.0 x 10 -7 = (1.0 x 10 -7)2 / (#M)[SO3

-2] = 0.024M

Kb = Kw/Ka = 1.0 x 10-14 / 1.0 x 10-7

= 1.0 x 10 -7

Kb = [HSO3-][OH-]/ [SO3

-2]

= (1.0 x 10 -7)2 / (#M - 4.90 x 10-5)

*Assume x (4.90 x 10-5) is so small so…

1.0 x 10 -7 = (1.0 x 10 -7)2 / (#M)[SO3

-2] = 0.024M

IV. 16. TITRATIONS

IV. 16. TITRATIONS

(Hebden p. 154-158)

(Hebden p. 154-158)

TitrationTitration•Process in which a measured amount of a solution is reacted with a known volume of another solution (one of the solutions has an unknown concentration) until a desired EQUIVALENCE POINT is reached

•Process in which a measured amount of a solution is reacted with a known volume of another solution (one of the solutions has an unknown concentration) until a desired EQUIVALENCE POINT is reached

Equivalence PointEquivalence Point

•The point in a titration where the ratio of the moles of each species involved exactly equals the ratio of the coefficients of the species in the balanced rx equation

•The point in a titration where the ratio of the moles of each species involved exactly equals the ratio of the coefficients of the species in the balanced rx equation

Example:Example:

•If 25.9 mL of H3PO4 with an unknown molarity react with 34.6 mL of 0.400 M KOH according to the rx, what is the molarity of the H3PO4?

•If 25.9 mL of H3PO4 with an unknown molarity react with 34.6 mL of 0.400 M KOH according to the rx, what is the molarity of the H3PO4?

H3PO4 + KOH K3PO4 + H2OH3PO4 + KOH K3PO4 + H2O

25.9 ml 34.6 ml # M 0.400 M

25.9 ml 34.6 ml # M 0.400 M

% Yield% Yield

= mass of product obtained x 100%mass of product expected

= mass of product obtained x 100%mass of product expected

% Purity % Purity

= mass of pure reactant x 100%mass of impure reactant

= mass of pure reactant x 100%mass of impure reactant