3 Ways to Measure Solution Strength.
Molarity = Moles of solute/Liters of solution.
Units: mol/L
Abbreviation: M
Example:
WHAT IS THE MOLARITY OF A SOLUTION THAT HAS 200 g
OF MgCl_{2}
IN
1500 ml OF SOLUTION?
Remember
in means 'divided by', so top and bottom of 1st
factorlabel cell must be used. Factorlabel
setup
200g
MgCl_{2} 
1000mL 
1
mol 
1500mL 
1L 
95.21g
MgCl_{2} 
= 1.4 M
* When mixing solutions. Weigh calculated mass of dry chemical. Add it to about 3/4 of the needed water. Then very carefully add water to exactly the needed amount. Usually in a volumetric flask.
Molality: moles of solute/ kilogram of solvent.
Units: mol/Kg
Abbreviation: m
(little m, not big M)
Example:
WHAT IS THE MOLALITY OF A SOLUTION THAT HAS 5.2
MOLES OF LEAD CHROMATE IN 2000 g OF WATER?
Factor label setup
5.2 mol
PbCrO_{4} 
1000g 
2000g
H_{2}O 
1
Kg 
=2.6 mol/Kg or 2.6 m
Harder example:
WHAT IS THE MOLALITY OF A SOLUTION MADE BY
DISSOLVING 45.0 GRAMS OF DEXTROSE, C_{6}H_{12}O_{6},
IN
500
GRAMS OF WATER?
45g
Dextrose 
1000g
H_{2}O 
1 mol
Dextrose 
500g
H_{2}0 
1
Kg H_{2}0 
180 g
Dextrose 
= 0.5 m
Molality is really useful when solvent has a different density than 1.0 g/mL, like with organic solvents.
Molality also controls freezing point depression, boiling point elevation.
Normality: equivalents of solute/liter of solution
Units:eq/liter
Abbreviation: N
Added step to factor label problems: # equivalents/mole, or number of positive
charges/mole
Example:
HOW MANY GRAMS OF ZINC NITRATE WOULD
YOU NEED TO MAKE 1.7 LITERS OF 0.6 N SOLUTION?
Remember N really stands for 0.6 eq/L so it is a compound unit.
Start the factor label problem with the single unit 1.7L !
Factor label setup
1.7 L 
0.6
equivalents 
1 mol 
189.4 g
Zn(NO_{3})_{2} 

1 L 
2
equivalents 
1 mol 
Zn is +2 so 2 positive charges or 2eq/mole of substance
= 96.6 g
*This
step in
red is somewhat
tricky.To find the equivalents per mole of substance look at the compound
formula. Find the cation. (Zn^{2+})
multiply by the subscript (unwritten 1), 2 x 1 = 2 equiv./mole of compound.
Why be normal?
Different compounds bring with them different numbers of positive charges. With phosphoric acid
(H_{3}PO_{4}
you get 3 positive charges per mole! With HCl you only get
1. So if you had and an acid spill and were neutralizing it with NaOH (1
positive charge per mole) you would need 3 times as much to neutralize
phosphoric acid as it would take to neutralize the hydrochloric acid. Normality
is for neutralization.
2. Remember, identify the cation (+ ion, not part of the polyatomic anion
though : ] ). Write the charge down. Multiply by the subscript. This gives you
equivalents/mole of the compound. 