Notesale: Turn your study into money

Document Preview

Extracts from the notes are below, to see the PDF you'll receive please use the links above

---

Titration and mole concept: Finding unknown concentrations

Introduction:

In the study of Chemistry solving mathematical problems has always proved to be

difficult for many students
...
What is it? How do you work with it?

A mole is a quantity of substance with Avogadro's number of particles in it
...

Avogadro's number is 6
...


In this lesson, a very simple presentation of the concept of the mole, using first principle,
is introduced to you
...
If you

know the cost of 5 pencils is 25 cents, then the cost of 1 pencil is 25/5 cents ( 5 cents)
...
That is, the cost of 1 pencil times 12
...

In the example used above a relationship between number of pencils and cents was
used
...
one solution, most commonly acid with a known concentration, is used
...
The unknown concentration of the alkali

solution can be calculated by using the end point volume and the known concentration

of the acid solution
...


The end point is that point at which just enough acid is neutralized by the alkali (or vice

versa)
...


Universal, Methyl orange, Screened methyl orange and phenolphthalein are common
indicators
...
The principles shown in the example will awaken your

understanding
...


Then there are work-on-your own problems with answers which you will work toward
...
This principle of relationships
will be able to be applied to all other problems involving the mole and will produce
successful results
...


A titration is a chemical process s in which the unknown concentration of one solution is
measured by known concentration of another solution
...
The mole is a unit of quantity of substance
...
023 x 10 23

Unit of concentration is in mol dm-3
...
This is the number of
moles of a solute dissolved in 1000 cm3 ( 1 dm3) of a solution
...


2 mol dm-3 means that there are 2 moles of substance in 1 dm3 of solution
Since 1 dm3 = 1000 cm3 then 2 mol dm-3 also mean that
there are 2 moles of substance in 1000 cm3 of solution
...
Here the

relationship is between the substance you are told about and the substance you are
asked about
...


Example:
When 20 cm3 of 2 mol dm-3 (concentration) HCl were titrated with sodium hydroxide

solution, it took 22 cm3 of the base to neutralize the acid
...
Determine which substance information
is given about and which substance information is asked about
...


Hydrochloric acid HCl

Sodium Hydroxide is NaOH

Sodium Chloride is NaCl

Water is H 2O

(d) write a balanced chemical equation between the acid and base
...

(ii) Who are you asked about?

sodium hydroxide
...

What is the relationship between the two substances, hydrochloric acid and sodium
hydroxide?

The relationship is shown in the mole ratio between HCl and NaOH
...
This means that 1 mole of HCl reacted
with 1 Mole of NaOH to produce 1 Mole of NaCl and 1 mole of water H 2O
...

This means that 1 mole of hydrochloric acid will completely neutralize 1 mole of sodium
hydroxide
...
5 mole of hydrochloric acid then only 0
...


If 2 moles of hydrochloric acid are used then it will take 2 moles of sodium hydroxide to
neutralize it
...

(iii) Now, to upgrade the table that will lead to the final answer:
Hydrochloric acid

Sodium Hydroxide

1:

1

Volume used

20 cm3

Number of moles

A

Mole ratio

Concentration

2 mol dm-3 ( the starting
point)

22 cm3

B

C

There are three pieces of information missing in this table:
(i) The number of moles of hydrochloric acid in 20 cm3 of 2 mol dm-3 of hydrochloric
acid, A

(ii) the number of moles of sodium hydroxide, B, needed to neutralize 20 cm 3 of 2 mol
dm-3 of hydrochloric acid

(iii) the concentration of the sodium hydroxide, C, which is the final answer for this
problem
...

To find A, we must start with what is known about Hydrochloric acid:
From its concentration of 2 mol dm-3 we know that
1000cm3 of hydrochloric acid contains 2 moles of it
Therefore, 1 cm3 of hydrochloric acid contains 2/1000 mole of it
Therefore 20cm3 (the volume used) of hydrochloric acid contains 2/1000 x 20 mole of it

This is equal to 0
...
04 mole of HCl will be neutralized by 0
...

This means that 22 cm3 of sodium hydroxide ( the volume of sodium hydroxide used at
the end point) contained 0
...
0
...

We now know the values of A and B
...
This is what concentration is about
...
04 mole of it
...
04/22 mole of it
And 1000 cm3 of sodium hydroxide solution will contain 0
...
82 mol of sodium hydroxide
...
Since 1000 cm3 of the sodium hydroxide solution
contained 1
...
82 mol dm-3
...


Now practice these steps with these problems:
A
...
5 mol dm-3 (concentration) Nitric acid HNO3 was titrated with

sodium hydroxide solution, it took 20 cm3 of the base to neutralize the acid
...
5 mol dm-3 ( the starting
point)

about)

1

B

C

B
...
What was the concentration of
the sodium carbonate solution?

Remember: Acid + metal carbonate → salt + water + carbon dioxide
Help!! hydrochloric acid react with sodium carbonate to produce sodium chloride salt,
water and carbon dioxide
...
Calculate the concentration of the Potassium hydroxide solution if 20 cm 3 of it
completely neutralized 15 cm3 of 2 mol dm-3 of hydrochloric acid
...
5 mol dm -3
2
...
2 mol dm-3 of sodium carbonate solution
...
56 mol dm -3



---