Molarity: Molarity is the concentration of a solution expressed as the number of moles of solute per litre of solution.
Explanation:
To get the molarity, you divide the moles of solute by the litres of solution.
For example, a 0.25 mol/L NaOH solution contains 0.25 mol of sodium hydroxide in every litre of solution.
To calculate the molarity of a solution, you need to know the number of moles of solute and the total volume of the solution.
To calculate molarity:
- Calculate the number of moles of solute present.
- Calculate the number of litres of solution present.
- Divide the number of moles of solute by the number of litres of solution.
EXAMPLE:
What is the molarity of a solution prepared by dissolving 15.0 g of NaOH in enough water to make a total of 225 mL of solution?
Solution:
Some students prefer to use a “molarity triangle”.
Units:
In the International System of Units (SI) the coherent unit for molar concentration is mol/m3. However, this is inconvenient for most laboratory purposes and most chemical literature traditionally uses mol/dm3, which is the same as mol/L. This traditional unit is often denoted by the letter M, optionally preceded by an SI prefix as needed to denote sub-multiples, for example:
mol/m3 = 10−3mol/dm3 = 10−3mol/L = 10−3 M = 1 mmol/L = 1 mM.
The units millimolar and micromolar refer to mM and μM (10−3 mol/L and 10−6 mol/L), respectively.
| Name | Abbreviation | Concentration | |
|---|---|---|---|
| (mol/L) | (mol/m3) | ||
| millimolar | mM | 10−3 | 100 |
| micromolar | μM | 10−6 | 10−3 |
| nanomolar | nM | 10−9 | 10−6 |
| picomolar | pM | 10−12 | 10−9 |
| femtomolar | fM | 10−15 | 10−12 |
| attomolar | aM | 10−18 | 10−15 |
| zeptomolar | zM | 10−21 | 10−18 |
| yoctomolar | yM | 10−24 (6 particles per 10 L) | 10−21 |
