Moles, Mass and Mr

Chemists count tiny particles using the mole, like using a dozen but much larger. One mole contains Avogadro’s number of particles, $N_A = 6.02 \times 10^{23}$. The mole links how many particles there are to the mass you can measure on a balance.

Key ideas

• Relative atomic mass (Ar): the number on the Periodic Table for each element (no units).
• Relative molecular/formula mass (Mr): add up the Ar values of all atoms in the formula (no units).
• Molar mass: the mass of 1 mole of a substance, in g/mol. Numerically, molar mass = Mr.

Core formulas

$$n \, (\text{mol}) = \frac{\text{mass (g)}}{M_r}$$

$$\text{mass (g)} = n \times M_r$$

$$\text{number of particles} = n \times N_A$$

Also, $M_r = \sum A_r$ (sum of the atoms’ Ar values).

Finding Mr quickly

• H₂O: $2\times1.0 + 16.0 = 18.0$ → $M_r = 18.0$
• NaCl: $23.0 + 35.5 = 58.5$ → $M_r = 58.5$
• CO₂: $12.0 + 2\times16.0 = 44.0$ → $M_r = 44.0$

Analogy: A recipe might say “2 cups flour per cake.” In chemistry, $M_r$ plays the role of “how heavy one ‘cup’ is,” and moles tell you how many ‘cakes’ (amounts) you have for a given mass.