Conservation of Mass and Stoichiometry
Aligned to HS-PS1-7 — Next Generation Science Standards.
What this lesson teaches
Atoms are not created or destroyed in a chemical reaction — they are just rearranged. That is why a balanced equation must have the same number of each type of atom on both sides, and why the total mass of reactants equals the total mass of products.
Worked examples
Example 1
Balance CH4 + O2 -> CO2 + H2O. Carbon balances (1 each). Hydrogen: 4 on the left needs 2 H2O on the right. Oxygen: the right now has 2 + 2 = 4 O atoms, so use 2 O2 on the left. Balanced: CH4 + 2 O2 -> CO2 + 2 H2O, with 1 C, 4 H, and 4 O on each side.
Example 2
Balance by changing coefficients only. Rewriting H2O as H2O2 balances the equation and changes water into hydrogen peroxide.
Example 3
Mass is conserved even when it looks lost. Burn 1 g of methane in 4 g of oxygen and you get exactly 5 g of products, most of it invisible gas.
Practice questions
- Why must a chemical equation be balanced?
- Balance the equation: H2 + O2 -> H2O.
- If 12 g of carbon reacts completely with 32 g of oxygen to form carbon dioxide, what is the mass of carbon dioxide produced, and which law tells you so?
Watch the lesson
Every lesson comes with a video taught in English and Spanish — the same video the QR code in the printed workbook opens.
▶ Watch this lessonEn español
Conservación de la masa y estequiometría
Los átomos no se crean ni se destruyen en una reacción química: solo se reordenan. Por eso una ecuación balanceada debe tener el mismo número de cada tipo de átomo en ambos lados, y por eso la masa total de los reactivos es igual a la masa total de los productos.
Ejemplo 1: Balancea CH4 + O2 -> CO2 + H2O. El carbono se equilibra (1 en cada lado). Hidrógeno: 4 a la izquierda requiere 2 H2O a la derecha. Oxígeno: la derecha ahora tiene 2 + 2 = 4 átomos de O, así que usa 2 O2 a la izquierda. Balanceada: CH4 + 2 O2 -> CO2 + 2 H2O, con 1 C, 4 H y 4 O en cada lado.
Ejemplo 2: Balancea cambiando solo los coeficientes. Reescribir H2O como H2O2 balancea la ecuación y convierte el agua en peróxido de hidrógeno.
Ejemplo 3: La masa se conserva aunque parezca perderse. Quema 1 g de metano en 4 g de oxígeno y obtienes exactamente 5 g de productos, casi todo gas invisible.
More Matter and Its Interactions lessons
Teach this lesson today
An AI tutor that guides with questions instead of just giving answers — all K–12 core subjects, every child in the family.
Start free — no card required