If Acid and Base Then Reaction Unless a Buffer

ACID BASE HELP SHEET

Note – this guide is primarily written in order to help you determine the pH of various acid base scenarios. If pH is given, then you will have to do some thinking and figure out how to work “backwards” through the steps in order to determine concentration or K values as necessary.

1) STRONG ACID

a) For a strong acid [H3O+] = [acid]

b) Determine pH directly from the concentration of the acid.

2) STRONG BASE

a) For a strong base such as M(OH)z -- [OH-] = z[base]

b) Determine pOH directly from the concentration of the base multiplied by z.

c) Now determine pH.

d) For metal oxides [OH-] = 2[metal oxide]; now get pOH and pH.

3) WEAK ACID

a) Write acid equilibrium equation

b) Perform ice box problem to determine pH.

c) If pH is given in the problem, then you can use ice box to determine the concentration of the acid

or the Ka value as necessary.

4) WEAK BASE

a) Write base equilibrium equation

b) Perform ice box problem to determine pOH and then get pH.

c) If pH is given in the problem, then you can get pOH. Then use ice box to determine the

concentration of the base or the Kb value as necessary.

5) STRONG ACID + STRONG BASE (reaction occurs)

First, you must write the balanced equation for the reaction. Calculate the mol (or mmol) of

each reactant. Then do stoichiometry to determine which reactant is limiting and which is excess.

a) Determine new concentration of the excess reactant by dividing moles (or mmol) of remaining

excess reactant by the new volume.

b) Determine the pH or pOH from this excess reactant concentration.

6) BUFFER –

a) WEAK ACID + CONJUGATE BASE OF WEAK ACID (no reaction occurs)

1) Write the acid equilibrium equation

2) Perform ice box problem which includes the concentration of the weak acid and of the

conjugate base, and determine the pH.

b) WEAK BASE + CONJUGATE ACID OF WEAK BASE (no reaction occurs)

1) Write the base equilibrium equation.

2) Perform ice box problem which includes the concentration of the weak base and of the

conjugate acid, and determine the pOH and then get the pH.

7) STRONG BASE + WEAK ACID (reaction occurs)

First, you must write the balanced equation for the reaction. Calculate the mol (or mmol) of

each reactant. Then do stoichiometry to determine which reactant is limiting and which is excess.

a)Weak acid is excess –

1) Do stoichiometry to determine how many moles (or mmol) of weak acid are left over and how many moles (or mmol) of conjugate weak base are formed.

2) Calculate the new concentration of each by dividing by new volume.

3) Now do appropriate weak acid ice box buffer problem to determine pH. (#6a)

b)No excess or limiting reactant –

1) Do stoichiometry to determine how many moles (or mmol) of conjugate base are formed.

2)Calculate the concentration of the conjugate base by dividing by the new volume.

3)Now do appropriate weak base ice box problem to determine pOH and then get pH. (#4)

c)Strong base is excess –

1) Do stoichiometry to determine how many moles (or mmol) of strong base is left over.

2) Calculate the new concentration of strong base by dividing by new volume.

3) Now determine pOH and then get pH. (ignore formation of weak conjugate base) (#2)

8) STRONG ACID + WEAK BASE (reaction occurs)

First, you must write the balanced equation for the reaction. Calculate the mol (or mmol) of

each reactant. Then do stoichiometry to determine which reactant is limiting and which is excess.

a) Weak base is excess –

1) Do stoichiometry to determine how many moles (or mmol) of weak base are left over and how many moles (or mmol) of conjugate weak acid are formed.

2) Calculate the new concentration of each by dividing by new volume.

3) Now do appropriate weak base ice box buffer problem to determine pOH and then

get pH. (#6b)

b) No excess or limiting reactant –

1)Do stoichiometry to determine how many moles (or mmol) of conjugate acid are formed.

2)Calculate the concentration of the conjugate acid by dividing by the new volume.

3)Now do appropriate weak acid ice box problem to determine pH. (#3)

c) Strong acid is excess –

1) Do stoichiometry to determine how many moles (or mmol) of strong acid is left over.

2) Calculate the new concentration of strong acid by dividing by new volume.

3) Now determine pH. (ignore formation of weak conjugate acid) (#1)