Moles and Stoichiometry
Master the mole concept -- the bridge between atoms and grams -- and learn how to use balanced chemical equations to calculate quantities in chemical reactions.
The Mole Concept
Atoms and molecules are far too small and numerous to count individually. The mole (symbol: mol) is the SI unit for the amount of substance. One mole contains exactly 6.022 × 1023 particles (Avogadro's number, NA). This applies to atoms, molecules, ions or any specified particles.
Avogadro's Number
6.022 × 1023 mol-1
The number of particles in one mole of any substance
Molar Mass
The mass of one mole of a substance, in g mol-1.
Numerically equal to the relative atomic/molecular mass
Amount (moles)
n = m / M
moles = mass (g) divided by molar mass (g mol-1)
The Mole Triangle
n = m / M
Find moles from mass
m = n × M
Find mass from moles
M = m / n
Find molar mass
Balanced Equations and Mole Ratios
A balanced chemical equation shows the correct ratios of reactants and products. The coefficients in front of each formula tell us the mole ratio -- the relative number of moles of each substance involved.
Example: Combustion of Methane
CH4 + 2O2 → CO2 + 2H2O
1 mol CH4
2 mol O2
1 mol CO2
2 mol H2O
The coefficients give us the mole ratio: 1 : 2 : 1 : 2
Law of Conservation of Mass: In a balanced equation, the total mass of reactants equals the total mass of products. Atoms are rearranged, not created or destroyed. This is why we must balance equations.
Stoichiometric Calculations
Stoichiometry uses balanced equations and the mole concept to calculate the amounts of reactants consumed or products formed. The general approach follows a systematic method.
Problem-Solving Steps
Write the balanced equation
Convert given quantity to moles
n = m / M
Use the mole ratio
From the balanced equation coefficients
Convert moles to required quantity
m = n × M (or N = n × NA)
Common Molar Masses
Values in g mol-1
Key Vocabulary
Mole (mol)
The SI unit for amount of substance. One mole contains 6.022 × 1023 particles (Avogadro's number).
Molar Mass (M)
The mass of one mole of a substance, measured in g mol-1. Found by summing atomic masses from the periodic table.
Stoichiometry
The calculation of quantities of reactants and products in chemical reactions using balanced equations and mole ratios.
Mole Ratio
The ratio of moles of one substance to another in a balanced chemical equation, given by the coefficients.
Worked Examples
Calculate the number of moles in 44 g of CO2.
Molar mass of CO2: M = 12.0 + 2(16.0) = 44.0 g mol-1
Using: n = m / M = 44 / 44.0
n = 1.0 mol
How many molecules are in 2.0 mol of H2O?
Using: N = n × NA
N = 2.0 × 6.022 × 1023
N = 1.20 × 1024 molecules
In the reaction 2Mg + O2 → 2MgO, what mass of MgO is produced from 4.86 g of Mg?
Step 1: n(Mg) = m / M = 4.86 / 24.3 = 0.200 mol
Step 2: Mole ratio Mg : MgO = 2 : 2 = 1 : 1, so n(MgO) = 0.200 mol
Step 3: M(MgO) = 24.3 + 16.0 = 40.3 g mol-1
Step 4: m(MgO) = n × M = 0.200 × 40.3 = 8.06 g
Knowledge Check
Select the correct answer for each question. Click "Check Answer" to see if you are right.
Question 1
One mole of any substance contains approximately:
Question 2
What is the molar mass of H2O? (H = 1.0, O = 16.0)
Question 3
How many moles are in 80 g of NaOH? (Na = 23.0, O = 16.0, H = 1.0)
Question 4
In the equation N2 + 3H2 → 2NH3, the mole ratio of N2 to NH3 is:
Question 5
What mass of oxygen gas (O2) reacts with 2.0 mol of CH4 in: CH4 + 2O2 → CO2 + 2H2O?
Key Concepts Summary
- ●One mole contains 6.022 × 1023 particles (Avogadro's number).
- ●n = m / M relates moles, mass and molar mass.
- ●Molar mass (g mol-1) is found by summing atomic masses from the periodic table.
- ●Balanced equations provide mole ratios that allow stoichiometric calculations.
- ●Steps: balanced equation → convert to moles → use mole ratio → convert to required quantity.