1. Unit 7: Solution Chemistry
Section 1: Molarity and Molality

2. Solutions
Are a combination of at least two ingredients, such as water and salt
The substance in the larger quantity is the solvent
The substance in the smaller quantity is the solute
Are homogeneous mixtures
Don’t have to be liquids
Air is a solution composed of nitrogen (the solvent), oxygen, water vapor, carbon dioxide, and other gases
Brass is a solution composed of copper and zinc

3. Concentration
The higher the ratio of solute to solvent, the more concentrated the solution
Example: Kool-Aid can be as diluted or concentrated as you want it to be, just add more water to dilute the drink
The wrong concentration can cause an object to work improperly or be harmful to living things
Example: Eye drops for humans can be harmful for a canine because of the different concentration requirements

4. Saturation
When the solute and solvent are put together, there is a limit as to how much solute can be dissolved
Saturated solution: no more solute will dissolve at a specific temperature
Unsaturated solution: more of the solute can be dissolved at the same temperature
Supersaturated solution: an unstable and temporary situation where the solution contains more dissolved solute than it normally would
Occurs when there is a change in temperature, volume, or pressure

5. Molarity (M) The number of moles of solute per liter of solution
M = moles
liter
Deals with concentration

6. Molarity Example 1
What is the molarity of a solution made by dissolving 0.3 mole of Ca(NO3)2 in enough water to make 500 mL of solution?
M = moles
liter
= 0.3 mol
0.5 liter
= 0.6 M Ca(NO3)2

7. Molarity Example 2
How much 0.5 M NaCl can be made from 0.1 mole of NaCl?
M = moles
liter
0.5 M = 0.1 mole
x liters
0.5x = 0.1
x = 0.2 L NaCl

8. Molarity Example 3
What is the molarity of an Epsom salt (MgSO4) solution made by dissolving 6 grams of it in enough water to make 2 liters of solution?
Step 1: convert grams to moles
6 g MgSO4 x mol MgSO4 = 0.05 mol MgSO4
g MgSO4
Step 2: M = moles
liter
M = 0.05 mol = 0.03 M MgSO4
2 liters

9. Molarity Example 4
How many grams of KOH are needed in order to prepare 2 liters of a 0.25 M solution?
Step 1: M = moles
liter
0.25 M = x mol
2 L
x = 0.5 mol KOH
Step 2: convert moles to grams
0.5 mol KOH x g KOH = 30 g KOH
1 mol KOH

10. Molality (m) The number of moles of solute per kilogram of solvent
m = moles of solute
kilogram of solvent
Also deals with concentration, but used less frequently

11. Molality Example 1
Calculate the molality of an antifreeze solution made by dissolving 186 g of antifreeze (C2H6O2) in 2 kg of water.
Step 1: Convert grams to moles
186 g C2H6O2 x 1 mol C2H6O2 = 3.00 mol C2H6O2
g C2H6O2
Step 2: m = moles of solute
kilogram of solvent
m = 3.00 mol = 2 m C2H6O2
2 kg

12. Molality Example 2
How many grams of lye (NaOH) should be added to 800 g of water in order to make a 0.5 m solution?
Step 1: Convert g of solvent to kg of solvent
800 g x 1 kg = 0.8 kg
1000 g
Step 2: Solve for grams of solute
m = moles of solute
kilogram of solvent
0.5 m = x mol = 0.4 mol NaOH
0.8 kg
Step 3: Convert mol NaOH to g NaOH
0.4 mol NaOH x g NaOH = 20 g NaOH
1 mol NaOH