Overview
This lecture explains how to calculate the number of moles from a given mass using the concept of molar mass, and covers related calculations in chemistry.
Calculating Number of Moles
- The formula to calculate moles is: number of moles (n) = mass (m, in grams) ÷ molar mass (M, in g/mol).
- Molar mass is found using the periodic table by adding the atomic masses of all atoms in the compound.
- Always write the formula first in calculations for exam credit.
- Substitute the given values into the formula to solve for the unknown.
- Include the correct unit (mol) in your final answer.
Using the Formula Triangle
- The formula triangle helps remember relationships: n = m ÷ M, m = n × M, M = m ÷ n.
- Block out the variable you are solving for to see the operation you need (divide or multiply).
Example Problem: NaOH
- Calculate moles in 100 g of NaOH: find molar mass (Na: 23, O: 16, H: 1) = 40 g/mol.
- Substitute: n = 100 g ÷ 40 g/mol = 2.5 mol.
Rounding and Significant Figures
- Unless otherwise instructed, round final answers to two decimal places.
- Always check if the question specifies the number of decimal places or significant figures.
Calculating Mass from Moles
- If moles (n) and molar mass (M) are given, calculate mass: m = n × M.
- Example: For 3 mol of calcium carbonate (M = 100 g/mol), mass = 3 × 100 = 300 g.
Key Terms & Definitions
- Mole (n) — A unit representing 6.022 × 10²³ particles of a substance.
- Molar Mass (M) — The mass (in grams) of 1 mole of a substance, found on the periodic table (g/mol).
- Mass (m) — The amount of substance in grams or kilograms.
Action Items / Next Steps
- Practice using the n = m ÷ M formula with other compounds.
- Always write formula, show substitutions, and include units in all answers.
- Review rounding rules for significant figures.
- Watch linked videos for calculating molar mass and further stoichiometry practice.