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…

dissolving

solution

saturated

solubility

solute

solvent

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

Why solutes 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

Calculating concentrations

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.).

In a reaction I only need to use 250cm3 of this.

How many moles is that?

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.

How much have I used?

Moles used = volume used in cm3 x concentration 1000

QuestionsHCl is hydrochloric acid.

How many moles of HCl are

in:

a) 200 cm3 of 1M HCl?

b) 250 cm3 of 4M HCl?

c) 600 cm3 of 2M HCl?

d) 400 cm3 of 0.5M HCl?

e) 2500cm3 of 0.1M HCl?