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 example
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.
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: 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.
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