Some Basic Concepts of Chemistry
Easy Overview
Ever wondered how chemists count things that are way too small to see? That's what this chapter is about. You'll learn the mole concept — basically a way to count atoms and molecules like you'd count eggs in a carton. Stoichiometry is just the math that lets you predict how much stuff you'll get from a reaction.
Mole Concept — The Chemist's Counting Unit
Think of a dozen eggs — except instead of 12, we're counting 6.022 × 10²³ particles. That's one mole. Why such a weird number? Because that's how many atoms of carbon-12 you need to get exactly 12 grams of it. So a mole is just a bridge between the microscopic world (atoms) and the stuff you can actually weigh on a scale.
Molar Mass and the Mole Formula
Molar mass is just the mass of one mole of a substance. For carbon, it's 12 g/mol. For water, it's roughly 18 g/mol (16 for oxygen + 2 for hydrogens). The golden formula: n = m / M. n is number of moles, m is mass in grams, M is molar mass. That's basically all you need.
Percentage Composition
Ever looked at a nutrition label? Same idea. Percentage composition tells you what fraction of a compound's mass comes from each element. If water is 16 g oxygen out of 18 g total, that's about 88.9% oxygen by mass. Rest is hydrogen.
Empirical and Molecular Formulas
Empirical formula is the simplest whole-number ratio of atoms in a compound. Molecular formula is the real deal. Think of it like a recipe: empirical is the ratio of ingredients (2 eggs, 1 cup flour), molecular is the actual amounts for one batch.
Stoichiometry — Reaction Math
Stoichiometry is just balancing your checkbook, but with chemicals. If you know the balanced equation, you can figure out exactly how much of each reactant you need and how much product you'll get. The mole ratio from the balanced equation is your conversion factor.
Limiting Reagent and Yield
In a burger joint, if you have 10 buns but only 5 patties, you can only make 5 burgers. The patties are the limiting reagent. Same in chemistry. The reactant that runs out first determines how much product you get. Actual yield vs theoretical yield gives you percentage yield.
Key Points
- •One mole = 6.022 × 10²³ particles (Avogadro's number)
- •n = m / M — the single most important formula in this chapter
- •Empirical formula = simplest ratio; molecular formula = actual number of atoms
- •Balanced equations give you mole ratios — use them to convert between substances
- •Limiting reagent is the reactant that finishes first and limits product formation
- •Percentage yield = (actual yield / theoretical yield) × 100
- •Always balance the chemical equation before doing stoichiometric calculations
- •Molar volume at STP: 22.4 L for one mole of any gas
Practice Questions
- Calculate the number of moles in 22 g of CO₂. (Atomic masses: C = 12, O = 16)
- A compound contains 40% carbon, 6.67% hydrogen, and 53.33% oxygen. Find its empirical formula. If its molecular mass is 180 g/mol, find the molecular formula.
- In the reaction N₂ + 3H₂ → 2NH₃, if 28 g of N₂ reacts with 10 g of H₂, which is the limiting reagent? How much NH₃ is formed?
- What is the mass of one atom of carbon-12?
- Define molarity. Calculate the molarity of a solution containing 5.85 g of NaCl in 500 mL of solution.