M6 Lesson #5 Titration calculations  
Lesson #4

Topics
Neutralization reactions
Titration Curves
Titration Calculations
Titration Lab Technique

Titration Calculations

Chemists use the fact that the pH breaks sharply when an acid is neutralized by a base to accurately determine the concentration of either the acid or base solution. At this point their is stoichiometrically equal amounts of acid and base and the pH = 7. Since we know that

Moles of a substance = Concentration of solution (moles/L) x Volume (L)

Using the idea that there is stoichiometrically equal amounts of acid and base at the equivalence point and the formula above, we can develop yet another formula to calculate the concentration of the acid or base in the solution.

Balance Base (Bb)
x
Moles of Acid
=
Moles of Base
x
Balance Acid (Ba)
Bb x Ca x Va = Ba x Cb x Vb

Equivalence Point

The point at which the pH = 7 is called the equivalence point. This point can be determined within a few drops of base solution added, because the pH changes sharply near this point. Chemists use an indicator to signal the pH change. An indicator is a substance that changes colour with certain changes in pH. The experimenter picks a indicator that changes colour in the pH range that corresponds to the break in the titration curve. This will vary depending on what combinations of acids and bases are titrated. When the indicator changes colour the titration is said to have reached it's endpoint. If the titration is done properly the endpoint is very close to the equivalence point.

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Titration Calculations: Procedure

To calculate the concentration of the acid or base then chemists need to know the balances in the neutralization reaction, the volumes of the acid and base added together to reach a pH = 7 and the concentration of either the acid or base.

Important Procedure
pencil
  

Titration calculations
Step 1 Balance the neutralization equation
Determine Balance of Acid and Base.
 
Step 2 Determine what is information is given
Step 3 Determine what information is required.
Step 4 Solve using the equation below
Bb x Ca x VA = BA x CB x Vb
 

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Examples and Assignments

Calculate the concentration of a nitric acid solution (HNO3) if a 20 ml sample of the acid required an average volume of 55 ml of a 0.047 mol/l solution of BA(OH)2 to reach the endpoint of the titration.

Step 1:

2 HNO3 + Ba(OH)2 -----> Ba(NO3)2 + 2 H2O
Balance Base = 1
Balance Acid = 2

Step 2:

Given information
Volume Acid = 20 ml
Volume Base (average) = 55 ml
Concentration of Base = 0.047 mol/l

Step 3:

Required information
Concentration of Acid

Step 4:

Solve using the equation.
Bb x Ca x Va = Ba x Cb x Vb
1 x Ca x 20 ml = 2 x 0.047 mol/l x 55 ml
Ca = 0.2585 mol/l ( considering significant figures 0.26 mol/l)

 
 
 
 
 
 
 
 
 
 

 

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Assignments on Titration

a) Check your understanding of Titration calculations

b) Assignment: Titration Calculations

 

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