1.1.7 moles and solutions
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1.1.7 Moles and Solutions
• Calculate the amount of substance, in mol using solution volume and concentration
• Describe a solutions concentration using the terms concentrated and dilute
You will be able to…
A solution
solvent
solute
A solution is a mixture of a solvent and a solute.
The liquid is the solvent.
The solid, liquid or gas dissolved in that liquid is the solute.
Concentration
dilute concentrated
- small amount of solute in the solvent
- large amount of solute in the same amount of solvent
Concentration
saturated
- large amount of solute in the same amount of solvent, with some solute unable to dissolve
A water molecule, H2O
Because of the properties of the individual atoms there is an uneven distribution of the electrons within the molecule, they tend to cluster towards the oxygen atom
δ -
δ + δ +
The molecule is POLAR
If the solute is a positive ion, eg Na+
The polar water molecules cluster around the solute, thereby allowing it to dissolve
The two things that decide the concentration of a solution are:
― The number of particles of solute
― The amount of solvent it is dissolved in.
Numbers of particles are measured in moles. Amounts of solution are measured by the volume
used.
Volumes
Concentrations are measured per litre of solvent.
One litre of a solvent is 1000cm3.
Another unit for a litre is dm3 (‘decimetre cubed’.)
A decimetre is 10cm.
10 cm x 10 cm x 10 cm = 1000cm3
10 cm
10 cm
10 cm
Volumes
Concentrations are measured per litre of solvent.
One litre of a solvent is 1000cm3.
Another unit for a litre is dm3 (‘decimetre cubed’.)
When working out a concentration you must always convert the volume into dm3.
e.g. 200 cm3 = 200 = 0.2dm3
1000
Concentration
Concentration = number of moles ÷ volume
c n v
c
n
vx
Concentrations are measured in moles per decimetre cubed (mol/dm3.)
Moles per decimetre cubed (mol/dm3.) is sometimes written as M.
Questions
What is the concentration of:
a) 58.5g NaCl in 2000cm3 water?b) 5.85g NaCl in 2000cm3 water?c) 5.85g NaCl in 500cm3 water?d) 34g NH3 in 200cm3 water?
Atomic masses
Na = 23
Cl = 35,5
N = 14
H = 1
Answers
a) 0.5M (1 mole ÷ 2 dm3)
b) 0.05M (0.1 mole ÷ 2 dm3)
c) 0.2M (0.1 mole ÷ 0.5 dm3)
d) 10M (2 mole ÷ 0.2 dm3)
How much have I used?
Imagine this represents one dm3 of a 2M solution (2 moles in 1000cm3 of water.).
I have used 250/1000 cm3 of the solution.
This is ¼ of the total.
I would expect to get ¼ of the number of particles of solute in that.
There were 2 moles in the original 1000cm3.
¼ of 2 moles = 0.5 moles.
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