Grade 8

Conservation of mass, stoichiometric ratios

In a closed system, chemical reactions conserve total mass because atoms are rearranged, not created or destroyed. Balanced coefficients set mole ratios for quantitative calculations.

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IntuitionIntuition: particles and change

When vinegar reacts with baking soda in an open cup, gas escapes and the cup may seem lighter. In a sealed container, the escaping carbon dioxide remains part of the measured system.

Left: reactants. Right: products. The totals match in grams, in moles of each coefficient, and in atoms of each element.

SchoolSchool: models and rules

Definition: Key idea

The law of conservation of mass states that total mass remains constant in a chemical change when all matter is included in a closed system. Stoichiometry applies balanced-equation mole ratios to reactant and product amounts.

For methane combustion, $\ce{CH4 + 2O2 -> CO2 + 2H2O}$ . One mole CH₄ reacts with two moles O₂ to form one mole CO₂ and two moles H₂O.

Mass check for CH₄ + 2O₂ → CO₂ + 2H₂O
Species	Molar mass	Amount	Mass
CH₄	16 g/mol	1 mol	16 g
O₂	32 g/mol	2 mol	64 g
CO₂	44 g/mol	1 mol	44 g
H₂O	18 g/mol	2 mol	36 g

\sum m_{\mathrm{reactants}}=\sum m_{\mathrm{products}}\quad(\text{closed system})

Example: Worked example

How much O₂ reacts completely with 2.0 mol CH₄?

Solution

CH₄ + 2O₂ → CO₂ + 2H₂O gives the mole ratio 1:2, so 2.0 mol CH₄ requires 4.0 mol O₂.

Example: Second example

44 g CO₂ contains 12 g C and 32 g O. How much oxygen is in 88 g CO₂?

Solution

Doubling the mass doubles every component: 2 × 32 g = 64 g of oxygen — the ratio C:O in CO₂ is fixed at 12:32.

Mass check: 16 g CH₄ + 64 g O₂ = 44 g CO₂ + 36 g H₂O = 80 g. Coefficients are particle or mole ratios, not mass ratios; convert through molar masses.

In a sealed flask with reactants inside, the balance needle stays put during a reaction — the mass is conserved. If gas escapes to the air, the flask appears lighter, but including the escaped gas restores the total.

Example: Calculation / application example

When 2.43 g of magnesium ribbon burns completely in air, 4.03 g of white MgO powder is formed. How much oxygen reacted?

Solution

By conservation of mass in the reaction system: m(Mg) + m(O₂) = m(MgO). Thus m(O₂) = 4.03 g − 2.43 g = 1.60 g.