Overview
This lesson explains how to use the formula n = m/M to calculate moles, mass, and molar mass, using several worked chemical examples.
The Mole Calculation Formula
- The formula n = m/M is used, where n = number of moles, m = given mass (in grams), and M = molar mass (in grams per mole).
- Molar mass (M) is obtained from the periodic table and refers to the mass of one mole of a substance.
- Given mass (m) is the amount of the substance you have, measured in grams.
Example Problems
- For CO₂, with 88 g given: Carbon (12) + 2 × Oxygen (16) = 44 g/mol; 88/44 = 2 moles.
- For H₂O, with 54 g given: 2 × Hydrogen (1) + 1 × Oxygen (16) = 18 g/mol; 54/18 = 3 moles.
- For H₂SO₄, with 49 g given: 2 × Hydrogen (1) + Sulfur (32) + 4 × Oxygen (16) = 98 g/mol; 49/98 = 0.5 moles.
Rearranging the Formula (Triangle Method)
- A triangle helps remember formulas: m is at the top, n and M at the bottom.
- For mass: m = n × M.
- For moles: n = m/M.
- For molar mass: M = m/n.
More Example Applications
- Find the mass in 3 moles of CO₂: M = 44 g/mol; m = 3 × 44 = 132 g.
- Find the mass in 9 moles of NaOH: Na (23) + O (16) + H (1) = 40 g/mol; m = 9 × 40 = 360 g.
- Moles in 50 g CaCO₃: Ca (40) + C (12) + 3 × O (16) = 100 g/mol; n = 50/100 = 0.5 moles.
Key Terms & Definitions
- Mole (n) — A unit measuring the amount of substance; 1 mole contains Avogadro's number of entities.
- Given mass (m) — The actual mass of a substance provided, measured in grams.
- Molar mass (M) — The mass of one mole of a substance, found on the periodic table, in grams per mole (g/mol).
Action Items / Next Steps
- Practice more problems using the n = m/M formula in different ways.
- Memorize the common molar masses for elements.
- Review periodic table to quickly find molar masses.