Some Basic Concepts of Chemistry
Syllabus Unit: Physical Chemistry
Why it matters
Chapter overview
This topic builds the quantitative toolkit every other chemistry topic depends on: the mole as chemistry's counting unit, stoichiometry for scaling a balanced equation to real masses, empirical/molecular formulas for deducing a compound's actual composition, and the concentration terms used to describe solutions precisely.
1Mole Concept & Molar Mass
The mole is chemistry's counting unit — it converts the invisibly small (atoms, molecules) into a mass you can weigh on a balance.
Learning Objectives
- State Avogadro's number and define one mole.
- Calculate molar mass from a formula and convert between mass, moles and number of particles.
- Distinguish gram atomic mass, gram molecular mass and gram formula mass.
- Apply molar volume (22.4 L at STP) to relate moles of a gas to its volume.
Key Terms
The **mole** is the bridge between the atomic world (too small to weigh individually) and the macroscopic world (grams you can measure on a balance). It is simply a very large counting number, exactly like 'dozen' means 12.
Defining the mole
1 mole of any entity (atoms, molecules, ions, electrons) contains **6.022×10²³** of that entity — Avogadro's number (NA). Since 2019, the mole is an SI base unit defined by fixing NA to this exact value.
Molar mass and conversions
- Molar mass (M) = mass of 1 mole, in grams — numerically equal to atomic mass (for elements) or the sum of atomic masses (for compounds).
- Water, H2O: M = 2(1) + 16 = 18 g/mol. Carbon dioxide, CO2: M = 12 + 2(16) = 44 g/mol.
- Gram atomic mass applies to elements, gram molecular mass to molecular compounds, gram formula mass to ionic compounds (which don't exist as discrete molecules).
Molar volume of gases
At STP (0°C, 1 atm), one mole of any ideal gas occupies **22.4 L** — this lets you convert directly between moles of a gas and its volume without needing its identity.