1. Chapter Solutions

2. 12.3 – Energetics of Solution Formation
There are energy changes associated with the solution making process
We can view a solution forming in three steps, each with their own change in enthalpy

3. Step 1: Separating the solute
This step is always endothermic
Energy required to overcome forces holding solute particles together

4. Step 2: Separating the solvent particles
Also endothermic because energy required to overcome the intermolecular forces holding solvent together

5. Step 3: Mixing the solute with the solvent
This step is always exothermic, because the solute and solvent are now attracting each other
Due to intermolecular forces

6. Final result
The overall enthalpy change is the SUM of the 3 individual steps.
So the overall energy change depends upon exactly how endothermic the first two steps are, and how exothermic the final step is

8. 12.5 – Ways of Expressing Concentration**
Concentration of a solution can be expressed qualitatively or quantitatively
Terms dilute and concentrated are qualitative statements
Dilute meaning a relatively small concentration of solute
Concentrated meaning a relatively large concentration of solute
Our focus will be on the quantitative descriptions of concentration

9. ppm
Oftentimes, very dilute solutions are given in terms of parts per million (ppm)
ppm is given by
What this means
If I have a solution whose concentration is 1 ppm
It has 1 g of solute for every million (106) g of solution
Or 1 mg of solute per kg of solution

10. Because density of water is 1 g/mL
1 kg of a very dilute aqueous solution will have a volume very close to 1 L
So, 1 ppm also means 1 mg/L of solution
ppm often used to express maximum concentration of toxic or carcinogenic substances in the environment
In US, maximum arsenic in water is ppm

11. ppb
A solution that is VERY dilute is often measured in terms of parts per billion (ppb)
1 ppb means 1 g of solute per billion (109) g of solution
Or 1 microgram (μg) per liter of solution
So the allowable concentration of arsenic in water could also be 10 ppb.

12. Mole Fraction
Concentration can also be expressed by the number of moles of components of the solution.
Mole fraction we’ve talked about
No units, just a ratio
The sum of all of the mole fractions of the components adds up to 1

13. Molarity
The big one
Molarity (M) relates moles of solute to volume of solution
Very useful when doing stoichiometry, because of the easy mole relationship

14. Conversion of Concentration Units
Sometimes the concentration needs to be known in several different units
It is possible to convert concentration units

15. Example
A solution contains 5.0g of toluene (C7H8) and 225 g of benzene and has a density of g/mL. Calculate the molarity of the solution.

16. To find molarity, we need mol of toluene and volume of solution
We find volume by using the density of the solution
But first, we need total mass of solution

17. To find volume, we use density
Note: We actually have 230 g of substance
225 g benzene and 5 g of toluene
Finally, to find molarity, we just divide moles of toluene by liters of solution

18. Final Step – Almost There!
Note: To solve this problem we needed the following
Density (allows us to move from mass of solution to volume of solution)
Mass of solute (to find moles of solute)