C5c Concentration and Quantitative Analysis
What is concentration?
Concentration measures how crowded something is. A room with 50 people
in it is __________________ concentrated (crowded) than if it had just 10
people.
The ___________ concentrated a solution the more crowded the solute
particles.
What are the units of concentration and volume?
Concentration of solutions may be measured in:
1. (g per dm3)
2. (mol per dm3)
Volume is measured in dm3 or cm3
________ cm3 = 1 dm3.
Task
Practise converting cm3 to dm3 by filling in the tables below:
Volume
cm3 dm3
1000
10
575
0.5
25
17
2.5
0.013
0.0675
Calculating Concentrations
You must be able to do 5 types of calculation:
1. Convert the number of moles per decimetre cubed to the number of grams per decimetre cubed and vice versa
___________ = mol/dm3 _______________x_____________= g/dm3
Molar Concentration
mass g/dm3 mol/dm3
NaOH 40
NaOH 0.5
Na2CO3 212
Na2CO3 26.5
MgCl2 3.0
MgNO3 0.75
2. Calculate the concentration of a solution from the mass or number of moles of solute in a particular volume.
3. Perform simple calculations involving concentration, number of moles and volume of solution
concentration = _________ or concentration = __________
Example: 20g of sodium hydroxide, NaOH, was dissolved in 250cm3 of water.
a) What is the concentration of the solution in g/dm3
b) What is the concentration of the solution in mol/dm3
Not that sort of concentration!
4. Perform calculations involving concentration for simple dilutions of
solutions e.g. how to dilute a 1.0mol/dm3 solution into a 0.1 mol/dm3
solution.
Example 1
You are given a 1.0mol/dm3 solution of hydrochloric acid, HCl and some water.
How would you produce 500cm3 of a 0.1mol/dm3 solution?
STEP 1 Work out a ratio of the two concentrations
0.1/ 1.0 = 0.1
STEP 2 Multiply this ratio by the volume you want to end up with
0.1 x 500 = 50cm3
STEP 3 Work out the volume of water needed:
500cm3 – 50cm3 = 450cm3 water
5. Converting amounts of sodium to amounts of salt
If a slice of bread contains 0.2g of sodium, how much salt (NaCl) does it
contain?
1. Find the ratio of the molecular mass of Na : NaCl
2. Multiply this by the amount of sodium
divide the smallest number by the largest one
So 50cm3 is the volume of the original solution that you need to measure out
Note: foods may contain a natural source of sodium so the numbers on the packet are not 100% accurate.
Dilution
What is dilution?
If we add ______________ to a solution, the solute particles get ___________
apart. We say it has been ______________________.
A dilute solution has ___________________ solute molecules in one litre than a
concentrated solution.
What sort of substances do we dilute?
•
•
Why is dilution important?
food preparation -
medicines
baby milk
Using Food packaging to calclualte RDAs (recommended daily
allowances)
Food packaging contains nutritional information labels.
These tell you the amounts of _______________ in food
Recommended Daily Allowances (RDA’s) are the amounts of each nutrient that
are recommended for the average adult.
Converting amounts of sodium to amounts of salt
If a slice of bread contains 0.2g of sodium, how much salt (NaCl) does it
contain?
2. Find the ratio of the molecular mass of Na : NaCl
2. Multiply this by the amount of sodium
Note: foods may contain a natural source of sodium so the numbers on the packet are not 100% accurate.
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