Calculate pH for weak acid.

Example: (PSPM 2010/2011)

An amine, RNH2 is a weak base with a dissociation constant, Kb of 1.8 x 10-5. It was dissolved in water to form a dilute solution.

a) Write the dissociation equation for the base.

b) Calculate the pH of a 0.4 M aqueous solution of the amine. 

                                                                                                                                 
►►►Hint:
Step 1 : Weak base dissociation equation

                      RNH2(aq) + H2O(l)        →⟶   RNH3⁺(aq) + OH^‐(aq)

Step 2 : Derive the dissociation constant

Step 3: Write ICE table

	RNH2(aq) +	H2O(l)	RNH3+(aq)	OH‐(aq)
Initial (M)	0.4	-	0	0
Change (M)	-x	-	+x	+x
Equilibrium (M)	0.4-x	-	+x	+x

Step 3 : Solve to find [OH-]. Include any assumptions made

1.8 x 10^-5 =    (x)(x)

                     (0.4-x)

Checking = √Kb x 100   < 5%

                      [ ]i 

Kb << 1, thus 0.4-x = 0.4

1.8 x 10^-4 =    (x)(x)

                     (0.4)

x = 2.68 x 10^‐3M

[x] =[ OH^-] = 2.68 x 10^‐3M

Step 4 : Find the pH

pOH = ‐log [OH‐] = 2.57

pH = 14 ‐ pOH

= 11.43