Chapter 1 · Chemistry
Some Basic Concepts of Chemistry: Chapter Introduction
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One drop of water holds about 1.7 × 10²¹ molecules. The chapter shows how chemists measure matter and count its particles with the mole.
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Key points
- Matter: solid, liquid, gas; elements, compounds, mixtures.
- Measurement uses SI units and significant figures.
- Lavoisier: mass is neither created nor destroyed; such laws led Dalton to atoms.
- 1 mole = 6.022 × 10²³ particles, a counting unit like a dozen; it powers stoichiometry.
How many molecules in one drop?
A drop of water is about 0.05 g. Its molecules:
0.05 g ÷ 18 g mol⁻¹ = 2.78 × 10⁻³ mol; × 6.022 × 10²³ ≈ 1.7 × 10²¹ molecules
Counting one per second would take about 5 × 10¹³ years, far longer than the age of the universe (about 1.4 × 10¹⁰ years). So chemists count in moles.
The chapter in five steps
| Step | Idea |
|---|---|
| Matter | States (solid, liquid, gas) and kinds (elements, compounds, mixtures) |
| Measurement | SI units, significant figures |
| Laws | Conservation of mass (Lavoisier) and other laws of chemical combination → Dalton's atomic theory |
| Mole | 6.022 × 10²³ particles; molar mass |
| Stoichiometry | Amounts of reactants and products from a balanced equation |
Stoichiometry in one line
2H₂ + O₂ → 2H₂O (2 mol + 1 mol → 2 mol)
So 4 mol of H₂ needs 2 mol of O₂ and gives 4 mol of water.
Quick quiz
4 questions. Choose an answer for each, then submit to see your score and the explanations.