1. Calculating Concentrations
Unit 2

2. Concentration:
Definition: the amount of solute in a given amount of solution. (Recall: mL = 1L, 1000g = 1 kg)

3. When the concentration changes!
Dilution : Addition of solvent
= Reduced concentration
Dissolution: Addition of solute
= Increased concentration
Evaporation: Reduction of solvent

4. The Basics… Definition:
Concentration is the measurement of the amount of solute that is dissolved in a given amount of solution.
Recall: mL = 1L, 1000g = 1 kg

5. What are the units used to measure Concentration?
g/L
%(m/m), %(m/v), %(v/v)
ppm (parts per million)

6. Which of the following is most concentrated?

7. Which of the following is most concentrated?

8. Basic Formula m
Units: g/L C V

9. But how many decimal places should I keep?
A 120mL solution is made with 36g of salt. What is the concentration of the solution?
c = ?
v = 120mL = 0.120L
m = 36g
But how many decimal places should I keep?
Step 1: Always write down the variables. Do any conversions necessary.
To be answered next week!…

10. Can I solve for volume and Mass?
Formula for volume
Formula for mass
Multiply both sides by v
Multiply both sides by v
Divide both sides by C:
Cancel ‘v’ and solve for m:
Cancel ‘C’ and solve for v:

11. m C V

12. Let’s Practice: Back to Concentration
Mass
Volume
Concentration
24g
2.6L
5.54g
0.75L
1.50L
3.55g/L L
500g/L 2g
0.45g/L
0.25g
1.02g/L
9.2g/L
7.4g/L
5.33g
4.28g 4L
0.25L

13. 2) Find the mass of solute needed to prepare the following solution: 50mL of a 48g/L solution.
v = 50mL = 0.050L
m = ?
C = 48g/L
m = 0.050L x 48g/L
m = 2.4g
3) What quantity of solution would be required to prepare a 0.5g/L solution using 2.3 g of solute?
v =?
m = 2.3g
C = 0.5g/L
v = 5 L

14. Let’s Practice!
Transform the following Concentrations into g/L

15. How do I make a solution?
Determine (if necessary) how much solute is required.
Using a balance, measure out the solute.

16. Add solute to the correct size volumetric flask.
This line indicates exactly the volume of the flask: In most cases 100mL or 50mL
Add some water. Stir solution and make sure all the solute is dissolved.
Continue adding water until the desired volume is reached. Remember the volume of the solution must be respected.

17. Practice: How would you prepare 600mL of a salt water solution at 14 g/L concentration?
m = 0.600L x 14g/L
m = 8.4g
Measure out 8.4g of salt on the balance.
Add the salt to a 600mL volumetric flask.
Add approximately 300mL of water and stir until all the solute is dissolved.
Add water until the total volume is 600mL.

18. Dilution
A dilute solution contains less solute than a more concentrated solution.
The stock solution refers to the solution which you use to make the dilution. It is the original solution.

19. C1 x V1= C2 x V2
C1 Concentration of the initial solution (stock solution)
V1 Volume of the initial solution (stock solution) you will require.
C2 Concentration of the new solution you need to make.
V2 The final volume of the solution you require after dilution.
C1 and C2 must have the same units as each other.
V1 and V2 must have the same units as each other.
The amount of water added = V2(the final volume) – V1(volume at the start)

20. Given 450mL of a calcium chloride water solution with a concentration of 10 g/L, what will be its concentration if we add 50mL of water to the solution?
You are given a 15g/L stock solution and want to make 300mL of a 10g/L solution. How much of the stock solution will you need?
V1 = 450mL
C1 = 10g/L
V2 = 500mL
C2 = ?
V1 = ?
C1 = 15g/L
V2 = 300mL
C2 = 10g/L