Various Concentration Terms
Concentration is defined as amount of solute per amount of solution or solvent. The "amounts" you
chose is what gives each concentration term it's own uniqueness.
-
percent conc = (mass of solute)/(mass of solution) x 102
- ppm = (mass of solute)/(mass of solution) x 106
- ppb = (mass of solute)/(mass of solution) x 109
- molarity = (mol of solute)/(liters of solution)
For the rest of this page, concentration is assumed to be in MOLARITY.
Molarity
-
Concentration is often depicted with square brackets around the solute of interest.
concentration of hydrogen ion is depicted as [H+]. The concentration is
assumed to be molarity (mol/L or mmol/mL).
-
Concentration is also sometimes shown as molarity with an italicized capital "M" and a subscript for the solute.
The concentration of hydroxide would be MOH-. Unfortunately, italics are not always
used - sometimes, like here,
the non-italized version just looks better (MOH-).
Dilutions - adding more solvent (ie. water) to an already existing solution
M1·V1 = M2·V2
ANY concentration and volume unit will work here as long as the same units are
on BOTH sides of the equation.
Titrations (1:1 ratios)
Macid·Vacid
= Mbase·Vbase
NOTE: This equation only works for MONOprotic acids (one proton in the formula)
and monobases (one hydroxide in the formula). Why? Because then the moles of acid
will equal the moles of base, a nice 1:1 ratio. If you have ANY other ratio (not
1:1) you must use the following formula:
Titrations (any ratio)
| (Macid · Vacid) |
|
(Mbase · Vbase) |
|
= |
|
| nacid |
|
nbase |
Where the "n" is the number of moles of acid and base in the BALANCED
chemical equation.
More Volumetric Stuff - getting moles or millimoles
For a given volume of any solution, you can always calculate the number of
moles of solute...
(Msolute Vsolution) = moles of solute =
nsolute
To get moles you MUST use molarity for the concentration and liters as the volume.
If you use milliliters (mL) for volume you will get millimoles (mmol) for the
amount. Be careful and note which type unit you are using. Like in class, I
often prefer to use mL and get mmol. Just make sure you know what you are using.
If you pour together multiple solutions of the same solute, you must calculate
the number of moles in each separate solution first, then add all the moles
together, and divide by the total combined volume. As long as you quantify all
the mole sources (whether solutions, solids, or whatever) and the total accumulated
volume, you can easily calculate the concentration.
Mix solutions A, B, and C (all different concentrations and volumes, but all
the same solutes)
new molarity = (MA VA + MB VB
+ MC VC) / (VA + VB + VC)
Note that the numerator here is just a total of all the moles of solute and
the denominator is a total of all the volumes.
Acid/Base Formulas
Be sure and know how to get any one of these values from any one of the others...
| pH = -log[H+] |
| pOH = -log[OH-] |
|
| [H+] = 10-pH |
| [OH-] = 10-pOH |
|
| |
| Kw = [H+][OH-] |
| pH + pOH = 14 |
|
| Kw = 1.0 x 10-14 |
| |
|
the value for Kw is only valid at 25 C