Concentrations of Solutions (pg 128)

Question 1

What is concentration a measure of?

Answer 1

How crowded things are.

Lots of reactions in chemistry take place between substances that are dissolved in a solutuion. The amount of a substance (e.g. the mass of the number of moles) in a certain volume of a solution is call its concentration

The more solute (the substance thats dissolved) there is in a given volume, the more concentrated the solution.

Question 2

How can ‘Concentration’ be measured?

Answer 2

in g/dm³

1. one way to measure the concentration of a solution is by calculating the mass of a substance in a given volume of solution.

The units will be units of mass/units of volume.

Question 3

How do you calculate the concentration of a solution in g/dm³?

Answer 3

mass of solute (in g)
Concentration (in g/dm³) = ________________
Volume of solvent (in dm³)

Question 4

What is the concentration in g/dm³ of a solution of sodium chloride where 30g of sodium chloride is dissolved in 0.2 dm³ of water?

Answer 4

3.0
concentration = ___ = 150 g/dm³ (1 dm³ = 1000cm³)
0.2

Question 5

What’s the concentration, in g/dm³, of a solution with 15g of salt in 500 cm³?

Answer 5

Convert the volume to dm³ by dividing by 1000

500cm³ ÷ 1000 = 0.5dm³

Now you’ve got the mass and the volume in the right units, just stick them in the formula.

                            15 Concentration =  \_\_\_\_\_   =  30g/dm³
                             0.5

You can rearrange the equaion above to dind the mass of solute in a given volume of solution if you know its concentration. (look at the formula triangle to help with rearranging the equation on page 128)

Question 6

What mass of a solt would you need to dissolve in 0.40 dm³ of water to mae a solution with a concentration of 2 g/dm³?

Answer 6

Use the formula triangle (on page 128) to rearrange the equation to make mass the subject

Mass = concentration x volume

Use this equation to calculate the mass:

Mass = 24 x 0.40 = 9.6g

(remember all measurements have some uncertainty to them. For repeated measurements, you can calculate the average (mean) and also the range of your results (see diagram page 6). The range can be used to give you an idea of how uncertain the mean value is (see page 10).

Question 7

Calculate the concentration, in g/dm³, of a solution that contains 0.6g of salt in 15cm³ of solvent? (2 marks)

Answer 7

Volume = 15 ÷ 1000 =0.015dm³ ( 1 mark)

concentration = mass + volume = 0.6 + 0.015 = 40 g/dm³ (1 mark)

Question 8

what is g dm³?

Answer 8

Mass (g)

volume (dm³) (1 cubic decimetre = 1 Liter)