Chemistery

Stoichiometry sounds like the hardest word in the chapter and turns out to be the most mechanical part of it. It's a recipe calculation: if this much goes in, how much comes out? Once you learn one route through it, every problem uses the same route. The short answer: stoichiometry is using the coefficients of a balanced chemical equation as a mole ratio to work out how much of one substance reacts with, or is produced by, another. The coefficients compare moles , never grams — which is why every problem passes through moles on the way. What the coefficients actually tell you Take the reaction that makes ammonia: N₂ + 3 H₂ → 2 NH₃ Read it as a ratio: 1 mole of N₂ reacts with 3 moles of H₂ to make 2 moles of NH₃. That's the mole ratio, and it comes free with a balanced equation. What it does not say is that 1 gram of N₂ reacts with 3 grams of H₂. Different substances have different molar masses, so grams don't scale that way. Coefficients count particles, not we...

Glucose is written C₆H₁₂O₆ in your biology textbook and CH₂O in a chemistry problem — and both are correct. That's not a misprint; they're two different kinds of formula answering two different questions. The short answer: the empirical formula gives the simplest whole-number ratio of atoms in a compound. The molecular formula gives the actual number of each atom in one molecule. The molecular formula is always a whole-number multiple of the empirical formula. Quick comparison at a glance Feature Empirical formula Molecular formula What it shows Simplest atom ratio Real atom count per molecule Glucose CH₂O C₆H₁₂O₆ Hydrogen peroxide HO H₂O₂ Benzene CH C₆H₆ Water H₂O H₂O (identical) Found from Percent composition or mass data Empirical formula + molar mass Identifies the compound? No — many share one Narrows it down, but isomers share one Used for ionic compounds Yes (the formula unit) Not really — no molecules exist What is an empirical formula? The empirical...

You can't count molecules, but you can weigh them — and molar mass is the bridge between the two. It's the single most-used number in chemistry calculations, and it's read straight off a periodic table you already have. The short answer: molar mass is the mass of one mole of a substance, measured in grams per mole (g/mol) . Numerically it equals the substance's atomic or formula mass, so you find it by adding up the atomic masses of every atom in the formula. What molar mass actually means A mole is 6.022 × 10²³ particles. Molar mass answers the practical follow-up: what does that pile weigh? The periodic table tells you carbon's atomic mass is 12.011 . That means one mole of carbon atoms weighs 12.011 g . Oxygen's is 15.999, so a mole of oxygen atoms weighs 15.999 g. The system was built so those two numbers always match — no conversion factor, no extra step. Think of it like a price list. If eggs cost $3 per dozen, "per dozen" is your conversi...

These two words look alike, sound alike, and turn up in the same sentence constantly — so it's no surprise students swap them. But they describe completely different kinds of thing, and once you see the difference you'll never mix them up again. The short answer: a molecule is a particle — two or more atoms chemically bonded together. A mole is a counting unit — about 6.022 × 10²³ of something, the way "dozen" means 12. A molecule is a thing; a mole is an amount of things. Quick comparison at a glance Feature Mole Molecule What it is A unit of amount A physical particle What it tells you How many particles you have What the particle is made of Abbreviation mol Written as a formula (H₂O, CO₂) Size 6.022 × 10²³ particles Usually under a nanometre Everyday analogy "A dozen" "An egg" Can you see one? A mole of water is about 18 mL — yes No, far too small Example 1 mol of water = 6.022 × 10²³ water molecules One water molecule = 2 H + 1...

Paper doesn't burst into flame just sitting on your desk, even though burning it releases energy. Petrol won't ignite until a spark arrives. Why do reactions that give off energy still need a push to get going? The answer is activation energy — the hidden hurdle in front of every reaction. The short answer: activation energy (Eₐ) is the minimum amount of energy that reacting particles need in order to start a reaction — the energy required to break the initial bonds so new ones can form. It's an energy "hill" the reactants must climb before they can roll down to become products, which is why even energy-releasing reactions need a little energy to begin. Why reactions need a push For particles to react, they have to collide — and not just any collision works. They must hit each other hard enough (with enough energy) and in the right orientation. That minimum energy needed to start rearranging bonds is the activation energy . Picture pushing a boulder over a...

Chemists sort reactions into a few big families so you can predict what will happen. Two of the most common — and most opposite — are combustion and decomposition. One combines a fuel with oxygen and releases energy; the other tears a single compound apart. Tell them apart and you can predict the products at a glance. The short answer: in a combustion reaction, a fuel reacts rapidly with oxygen to release energy as heat and light, typically producing carbon dioxide and water. In a decomposition reaction, a single compound breaks down into two or more simpler substances, usually needing an input of energy such as heat. Combustion builds products by adding oxygen; decomposition breaks one substance apart. Quick comparison at a glance Feature Combustion Decomposition General pattern fuel + O₂ → CO₂ + H₂O (+ energy) AB → A + B Number of reactants Two (fuel and oxygen) One (a single compound) What happens Substances combine with oxygen One substance splits ap...