Understanding Molar Mass: The Key to Counting Particles by Mass
What Is Molar Mass?
Definition
Molar mass
Molar mass, denoted as $M$, is the mass of one mole of a substance.
- It is expressed in units of grams per mole ($\text{g mol}^{-1}$).
- But what exactly is a mole?
Definition
Mole
The mole (mol) is the SI unit for the amount of substance, defined as $6.02 \times 10^{23}$ particles (atoms, molecules, or ions).
- The number $6.02 \times 10^{23}$ is known as Avogadro’s constant.
- The molar mass tells you how much one mole of a substance weighs in grams.
- The molar mass of carbon ($C$) is $12.01 \, \text{g mol}^{-1}$.
- This means 1 mole of carbon atoms weighs $12.01$ grams.
- The molar mass of water ($H_2O$) is $18.02 \, \text{g mol}^{-1}$.
- This means 1 mole of water molecules weighs $18.02$ grams.
The Relationship Between Mass, Moles, and Molar Mass
- The relationship between mass, moles, and molar mass is captured by the formula: $$n = \frac{m}{M}$$ where:
- $n$ = number of moles (mol)
- $m$ = mass of the substance (g)
- $M$ = molar mass ($\text{g mol}^{-1}$)
- This formula allows you to calculate any one of the three variables if the other two are known.
Rearranging the Formula
- To find mass: $$m = n \times M$$
