1. Section 6.2—Concentration
How do we indicate how much of the electrolytes are in the drink?

2. Concentrated versus Dilute
solvent
solute
Lower concentration
Not as many solute (what’s being dissolved) particles
Higher concentration
More solute (what’s being dissolved) particles

3. Concentration
Concentration gives the ratio of amount dissolved to total amount
There are several ways to show concentration

4. Percent Weight/Volume
This is a method of showing concentration that is not used as often in chemistry
However, it’s used often in the food and drink industry
For example, your diet drink can might say you have less than g of salt in 240 mL.
That would give you a concentration of g / 240 mL, which is 0.015% solution

5. %(W/V) Example Example:
If you dissolve 12 g of sugar in 150 mL of water, what percent weight/volume is the solution?

6. %(W/V) Example 8.0% (W/V) Example:
If you dissolve 12 g of sugar in 150 mL of water, what percent weight/volume is the solution?
8.0% (W/V)

7. Concentration using # of molecules
When working with chemistry and molecules, it’s more convenient to have a concentration that represents the number of molecules of solute rather than the mass (since they all have different masses)
Remember, we use moles as a way of counting molecules in large numbers

8. Quick Mole Review 1 mole = 6.02 × 1023 molecules
The molecular mass of a molecule is found by adding up all the atomic masses in the atom
Molecular mass in grams = 1 mole of that molecule

9. How many moles are in 25.5 g NaCl?
Quick Mole Example
Example:
How many moles are in 25.5 g NaCl?

10. How many moles are in 25.5 g NaCl?
Quick Mole Example
Example:
How many moles are in 25.5 g NaCl? Na Cl 1
22.99 g/mole
35.45 g/mole  = +
58.44 g/mole
1 mole NaCl molecules = g
25.5 g NaCl 1
mole NaCl
= _______ mole NaCl
0.44
58.44
g NaCl

11. Molarity
Molarity (M) is a concentration unit that uses moles of the solute instead of the mass of the solute

12. If you dissolve 12 g of NaCl in 150 mL of water, what is the molarity?
Molarity Example
Example:
If you dissolve 12 g of NaCl in 150 mL of water, what is the molarity?

13. If you dissolve 12 g of NaCl in 150 mL of water, what is the molarity?
Molarity Example
Example:
If you dissolve 12 g of NaCl in 150 mL of water, what is the molarity? Na Cl 1
22.99 g/mole
35.45 g/mole  = +
58.44 g/mole
1 mole NaCl molecules = g
12 g NaCl 1
mole NaCl
= _______ mole NaCl
0.21
58.44
g NaCl
Remember to change mL to L! 150 mL of water = L
1.4 M NaCl

14. Converting between the two
If you know the %(W/V), you know the mass of the solute
You can convert that mass into moles using molecular mass
You can then use the moles solute to find molarity

15. Converting from % to M Example
What molarity is a 250 mL sample of 7.0 %(W/V) NaCl?

16. Converting from % to M Example
What molarity is a 250 mL sample of 7.0 %(W/V) NaCl?
? = 17.5 g NaCl Na Cl 1
22.99 g/mole
35.45 g/mole  = +
58.44 g/mole
1 mole NaCl molecules = g
17.5 g NaCl 1
mole NaCl
= _______ mole NaCl
0.30
58.44
g NaCl
Remember to change mL to L! 250 mL of water = L
1.2 M NaCl

17. Concentration of Electrolytes
An electrolyte breaks up into ions when dissolved in water
You have to take into account how the compound breaks up to determine the concentration of the ions
CaCl2  Ca Cl-1
For every 1 CaCl2 unit that dissolves, you will produce 1 Ca+2 ion and Cl-1 ions
If the concentration of CaCl2 is 0.25 M, the concentration of Ca+2 is 0.25 M and Cl-1 is 0.50 M

18. Let’s Practice #1 Example:
You want to make 200 mL of a 15% (W/V) solution of sugar. What mass of sugar do you need to add to the water?

19. Let’s Practice #1 30 g of sugar Example:
You want to make 200 mL of a 15% (W/V) solution of sugar. What mass of sugar do you need to add to the water?
30 g of sugar

20. What is the %(W/V) of a 500. mL sample of a 0.25 M CaCl2 solution?
Let’s Practice #2
Example:
What is the %(W/V) of a 500. mL sample of a 0.25 M CaCl2 solution?

21. What is the %(W/V) of a 500. mL sample of a 0.25 M CaCl2 solution?
Let’s Practice #2
Example:
What is the %(W/V) of a 500. mL sample of a 0.25 M CaCl2 solution?
? = moles CaCl2 Ca Cl 1 2
40.08 g/mole
35.45 g/mole  =
70.90 g/mole +
g/mole
1 mole CaCl2 molecules = g
0.125 moles CaCl2
110.98
g CaCl2
= _______ g CaCl2
13.9 1
mole CaCl2
2.8 %(W/V) CaCl2

22. Let’s Practice #3 Example:
What are the molarities of the ions made in a 0.75 M solution of Ca(NO3)2

23. Let’s Practice #3 Ca(NO3)2  Ca+2 + 2 NO3-1 Ca+2 = 0.75 M
Example:
What are the molarities of the ions made in a 0.75 M solution of Ca(NO3)2
Ca(NO3)2  Ca NO3-1
For every 1 Ca(NO3)2, there will be 1 Ca+2 and 2 NO3-1 ions
Ca+2 = 0.75 M
NO3-1 = 1.5 M