1. Ch. 15 Molarity vs. Molality
Mrs. Fox

2. Objectives
3221.T/E.1 Distinguish among tools and procedures best suited to conduct a specified scientific inquiry.
Identify properties of a solution: solute and solvent in a solid, liquid or gaseous solution: procedure to make or determine the concentration of a solution in units of ppm, ppb, molarity, percent composition, factors that affect the rate of solution.

3. What do we use molarity/molality for?
Show concentrations
Calculate how much solvent/solute needed to make a particular concentration
Use the symbols:
Molarity (M)
Molality (m)

4. Molarity vs. Molality moLarity = moles solute
L of solution
moLarity has only 1 L in its name
It can have an L in its formula
moLaLity= moles of solute
kg of solvent
has 2 Ls in its name
Can NOT have an L in its formula

5. Calculating molarity:
A 100.5mL (IV) solution contains 5.10g of glucose (C6H12O6). What is the molarity of this solution. The molar mass of glucose is g/mol
M= moles solute
L solution
To find moles:
5.10g C6H12O6 x 1 mole = mol glucose
180.16g
To find L:
“king henry danced once drinking chocolate milk”
K H D 1 D C M
you are going from “milli” to “1”(remember 1 means no prefix)
Move decimal 3 places to the left
100.5mL = .1005L solution
M= moles = M
0.1005L

6. Try some on your own:
What is the molarity of an aqueous solution containing 40.0g of glucose in 1.50L solution?
0.148M
How many moles are there in a 2.5L solution with a concentration of 3.0M?
7.5 moles

7. Calculating molality
In the lab, a student adds 4.5g of NaCl to 100.g of water. Calculate the molality of the solution:
m= moles solute
kg solution
To find moles:
4.5g NaCl x 1 mole = mole NaCl
58.44g
To find kg:
“king henry danced once drinking chocolate milk”
K H D 1 D C M
you are going from “1” to “kilo” (remember 1 means no prefix)
Move decimal 3 places to the left
100g = .100 kg H2O
m= moles = .77m
.100 kg

8. Try some:
What is the molality of a solution containing 10.0g Na2SO4 dissolved in g of water?
0.0704m
What is the molality of a solution containing 30.0g of naphthalene(C10H8) dissolved in 500.0g of toluene?
0.468m

9. Concentrated vs. Dilluted
What does it mean to dilute something?
To make something less concentrated
What does concentrated mean?
There are a lot of solute in the solvent
In saltwater, what is my solute?
“what YOU put in it”
Salt
In saltwater, what is my solvent?
“dissolves the solute”
Water
How can you make saltwater more concentrated?
Add more salt to the same amount of water
Evaporate some of the water out so that there is less water per salt
How can you make saltwater more diluted?
Add more water to the same amount of salt
Remove some of the salt out of the solution

10. In order to make dilutions use the formula: M1V1=M2V2
M= molarity (or moles/L) V= volume (L)
What volume, in mL of 2.00M CaCl2 stock solution would you use to make a 0.50L of 0.300M CaCl2 solution?
M1=2.00M
V1= ?
M2= 0.300M
V2= .50L
M1V1=M2V2
V1=M2V2/M1
V1=(.300 X .50)/(2.00)
V1=0.075L
Watch out! The question asks for mL, NOT L!!
KHD1DCM Move from the prefix “1” to “milli”
Move decimal 3 places left
0.075L = 75mL

11. Try Some:
What volume of a 0.300M KCl solution would you use to make .300L of a 1.25M KCl solution?
125 mL
If you dilute 20.0mL of a 3.5M solution to make mL of solution, what is the molarity of the dilute solution?
0.70M

12. Review:
If I have 50g of salt in 50L of water and I add 50L more water, is my solution now more concentrated or dilute?
Dilute
What formula would I use to find the concentration after I diluted this solution?
M1V1=M2V2
If I find mole/L did I find molality or molarity?
Molarity
How could I find the volume (L) in lab?
Graduated cylinder
How could I find the g in lab?
Weigh it on a scale
Which symbol stands for molality: M or m? M