Introduction
The mole is the chemist's counting unit — it lets us link the invisible world of atoms and molecules to things we can actually measure, like mass and volume. In this lesson we look at three key equations that let you convert between number of particles, mass, and gas volume, and the amount of substance in moles (n). These three formulas are the backbone of almost every stoichiometry calculation you will do this year (Talbot, p.418).
The three key equations
1. Moles from number of particles
n = N / N_A
- n = amount in moles
- N = number of particles (atoms, molecules, ions...)
- N_A = Avogadro's constant = 6.02 × 10²³ mol⁻¹
2. Moles from mass
n = m / M
- m = mass in grams
- M = molar mass in g mol⁻¹ (from the periodic table)
3. Moles from gas volume
n = V / V_m
- V = volume of gas
- V_m = molar volume of a gas (22.7 dm³ mol⁻¹ at STP, 100 kPa, 273 K)
Each formula can be rearranged to find any of the three variables, so it helps to remember them as a triangle: n is always "amount," and the other letter is whatever you're converting from.
Worked Example 1 – Using n = N/N_A
Question: How many moles are there in 3.01