Chemistry Week 24 - Day 3 - 4

Acid/Base Titration

Acid/Base Titration

the key points to useful information on this page

Titration is an analytical method in which a standard solution is used to determine the concentration of another solution.

Any solution for which the concentration is precisely known is called a standard solution.

An acid/base titration uses the fact that one can be "neutralized" with the other. In this neutralization reaction, the acid and base will combine to produce a salt plus water. When done correctly, the resulting solution will be "neutral" - neither acid nor base. In a titration, this is known as the end point. The change in pH of the solution can be monitored using an indicator or pH meter. It is extremely important that the exact amounts of each solution used be known at the end point.

A neutralization reaction

This balanced equation indicates that one mole of sodium hydroxide will combine with one mole of hydrochloric acid to produce one mole of sodium chloride and one mole of water. The only variable is the concentration of the two solutions.

Titration of a strong acid
with a strong base

Titration curve graph
This graph represents the titration of 10 ml of 0.1M HCl with 0.1M NaOH.

The end point is characterized by a rapid change in pH with very little base added.

Phenolphthalein is the indicator we will use for an acid/base titration. Add two drops to the acid solution to be titrated. The solution will be colorless at this point. Slowly add the base solution until the faint pink color persists.

At the end point you know the concentration of the standard and the volume used, as well as the volume of the unknown used. Calculations can now be done to compare the number of moles of each solution used. This will give you the concentration of the unknown.

Phenolphthalein Color Chart
colorless in an acid acidic solution.
faint pink in a neutral solution neutral solution.
dark pink in a base basic solution.

Use this webpagelink to an Internet Website to perform a virtual titration.

 

Titration Problem Example:

If 20 cm3 of a 0.3 M solution of NaOH is required to neutralize 30.0 cm3 of a sulfuric acid solution, what is the molarity of the acid solution?

Solution Steps:

  1. Write a balanced equation: 2NaOH + H2SO4 Na2SO4 + 2H2O
  2. Determine the number of moles of the standard NaOH solution used:
    0.005 moles of sodium hydroxide
  3. Use the mole ratio from the balanced equation
    to convert moles of NaOH to moles of H2SO4:
    0.003 moles of sulfuric acid
  4. Use the volume of acid solution used to determine the molarity of the acid solution:
    the concentration of the acid solution is 0.1M
    Notice that the   1dm3/1000cm3   and the   1000cm3/1dm3   will offset each other. You may shorten the problem by skipping these conversions.

Practice Problems:
calculate the unknown quantity for the complete neutralization of the following.

Acid Base
Concentration Volume Concentration volume
1. 0.25M HCl 30.00cm3 ? M NaOH
25.00cm3
2. 0.50M H2SO4
? cm3
0.75M KOH 20.00cm3
3. ? M HNO3
15.00cm3 1.50M NH3 25.00cm3
4. 0.400M HNO3 35.00cm3 0.800M NaOH
? cm3

 

Use this webtestlink to an Internet Website to practice titration calculations.

 

In-class Assignment 244:
This assignment must be turned in by the end of class tomorrow to receive credit.
Scoring criterialink to a local webpage

Work these titration calculations:

  1. A titration of 15.0 cm3 of household ammonia, NH3, required 38.57 cm3 of 0.78M HCl. Calculate the molarity of the ammonia.
  2. What volume of 0.5M HNO3 is required to neutralize 25 cm3 of a 0.2M NaOH solution?
  3. Calculate the volume of 0.6M HNO3 necessary to neutralize 28.55 cm3 of 0.45M KOH.

Research Links:

Acids and Bases

 

 

 

 

 

 

 

 

 

 

Answers to titration problems:

  1. 2M NH3
  2. 10 cm3 HNO3
  3. 21.4 cm3 HNO3