Acid Base 1 : Tut 1
Titration 1
Assume that you have a bottle of hydrochloric acid (HCl), of known concentration, and a
base like NaOH of unknown concentration.
There is an experiment you can do, and you can find out the base concentration.
You react the acid and base with each other, and check the volumes used.
Do a calculation, and you got the bases's concentration!
The above procedure is called a TITRATION.
Acids and bases have specific colours in indicators.
An indicator is a compound that is colour sensitive to pH values. In other words, the same indicator will display different colours in acids and bases.
When you are reacting the acid and base, you have to keep checking the colour they are creating in the beaker. When you get a neutral colour, you have the correct volumes added.
INDICATOR COLOUR TABLE..
SA = Strong Acid
SB = Strong Base
WA = Weak Acid
WB = Weak Base
Litmus is not used in titration experiments because it changes colour over too wide a pH range. With litmus you cannot tell when the reaction is really complete.
Only these indicators are used:
Seems a little scary.... relax, you get used to them.
You react the acid and base with each other, and check the volumes used.
Do a calculation, and you got the bases's concentration!
The above procedure is called a TITRATION.
Titration
A titration is a method where a solution of known concentration is used to determine the concentration of an unknown solution by an experiment.- known solution is called titrant
- unknown solution is called analyte
How does this work?
COLOURS!Acids and bases have specific colours in indicators.
An indicator is a compound that is colour sensitive to pH values. In other words, the same indicator will display different colours in acids and bases.
When you are reacting the acid and base, you have to keep checking the colour they are creating in the beaker. When you get a neutral colour, you have the correct volumes added.
INDICATOR COLOUR TABLE..
SA = Strong Acid
SB = Strong Base
WA = Weak Acid
WB = Weak Base
Litmus is not used in titration experiments because it changes colour over too wide a pH range. With litmus you cannot tell when the reaction is really complete.
Only these indicators are used:
- bromothymol blue
- methyl orange
- phenolphthalein
Reactions
You need to know some acid-base reaction equations.Seems a little scary.... relax, you get used to them.
Useful Equations
You don't necessarily use all these equations in the same example!- ca = concentration of acid
- Va = Volume of acid
- na = mols of acid
- cb = concentration of base
- Vb = Volume of base
- nb = mols of base
Important Definitions
- equivalence point:
the point at which the acid /base has completely reacted with the base/acid - endpoint:
the point where the indicator changes colour - hydrolysis:
the reaction of a salt with water - standard solution:
solution of known concentration - indicator:
compound that is colour sensitive to pH values
Let's do an example, that puts all the above together!
You have a solution of NaOH of unknown concentration, and you wish to determine this concentration. You perform a titration as shown in the diagram below, using a 0,22mol.dm-3 solution of HCl.
1. The acid is in one tube (X) and the base is in the other. What are the tubes called and what are used for?
burettes
used to dispense exact amounts of a solution
pipettes can also be used, but burettes are preffered for titrations
used to dispense exact amounts of a solution
pipettes can also be used, but burettes are preffered for titrations
2. What is the name for the flask Y?
Erlenmeyer flask (conical flask)
3. Is HCl and NaOH strong or weak?
HCl is a strong acid
NaOH is a strong base
NaOH is a strong base
4. Write down the equation for the reaction.
5. Explain why the salt will NOT undergo hydrolysis.
(Hydrolysis means to react with water)
(Hydrolysis means to react with water)
The salt NaCl is a neutral salt since it comes from a strong acid and a strong base. Hence it will not react with water, i.e. it will not undergo hydrolysis.
6. Which indicator would be most suitable for this titration?
bromothymol blue
7. What are the colours of HCl and NaOH in this indicator?
HCl : yellow
NaOH : blue
NaOH : blue
8. As you add the HCl and NaOH into the Erlenmeyer flask, the colours keep changing as the reaction proceeds.
What colour in the flask would let you know that you have reached the end point?
What colour in the flask would let you know that you have reached the end point?
green
an in-between colour of yellow and blue
If you held the test-tubes one behind the other, you can observe the colour you are looking for at end-point.
an in-between colour of yellow and blue
If you held the test-tubes one behind the other, you can observe the colour you are looking for at end-point.
9. At end point, you take the volume readings from the burettes, and record that you used HCl of volume 20cm3 and NaOH of volume 8cm 3.
Calculate the concentration of the NaOH.
Write out your equation, with the information like this (acid concentration was given as 0,22mol.dm-3) :
na and nb are the numbers you use to balance the equation.
Then use the formula:
na and nb are the numbers you use to balance the equation.
Then use the formula:
10 Burettes usually have a temperature, example, 250 C printed on them. Why is this?
Burettes are calibrated for a specific temperature. If the solution was hotter, expansion of the glass will cause incorrect volume readings since the burette would be larger.
Similarly, if the solution was too cold, the burette would have contracted, also giving incorrect readings since it would now be smaller.
Similarly, if the solution was too cold, the burette would have contracted, also giving incorrect readings since it would now be smaller.