Law of Conservation of Mass
What law says mass is not created or destroyed in a chemical reaction?
The Law of Conservation of Mass. Mass is not created or destroyed in a chemical reaction.
Where do coefficients go in a chemical equation?
Coefficients go in front of chemical formulas.
Balance: H₂ + Cl₂ → HCl
H₂ + Cl₂ → 2HCl
Balance: N₂ + H₂ → NH₃
N₂ + 3H₂ → 2NH₃
Balance: Al(OH)₃ + HBr → AlBr₃ + H₂O
Al(OH)₃ + 3HBr → AlBr₃ + 3H₂O
Coefficient ratio: 1:3:1:3
Why must each type of atom be equal on both sides of a balanced equation?
Atoms are rearranged, not created or destroyed. Equal atom counts show that the same matter is present before and after the reaction.
What does a coefficient tell you about a formula?
A coefficient tells how many molecules or formula units of a substance are represented.
Balance: Na + Cl₂ → NaCl
2Na + Cl₂ → 2NaCl
Balance: CH₄ + O₂ → CO₂ + H₂O
CH₄ + 2O₂ → CO₂ + 2H₂O
Balance: NaOH + H₂SO₄ → Na₂SO₄ + H₂O
2NaOH + H₂SO₄ → Na₂SO₄ + 2H₂O
A sealed reaction vessel has a total mass of 25 g before a reaction. What is the total mass afterward?
25 g. In a closed system, the total mass stays the same.
A student changes H₂O to H₂O₂ to balance an equation. What rule did the student break?
The student changed the compound’s formula. Balance equations by changing coefficients, not subscripts.
Balance: Mg + O₂ → MgO
2Mg + O₂ → 2MgO
Balance: Fe + O₂ → Fe₂O₃
4Fe + 3O₂ → 2Fe₂O₃
Balance: Ca(OH)₂ + H₃PO₄ → Ca₃(PO₄)₂ + H₂O
: 3Ca(OH)₂ + 2H₃PO₄ → Ca₃(PO₄)₂ + 6H₂O
A reaction in an open container seems to lose mass. Give one reason the measured mass could decrease without
: A gaseous product may have escaped from the open container. If all products are included in a closed system, total mass is conserved.
Why is changing a subscript different from changing a coefficient?
A subscript changes the identity and composition of a substance. A coefficient changes how many units of that substance are present.
Balance: KClO₃ → KCl + O₂
2KClO₃ → 2KCl + 3O₂
Balance: C₃H₈ + O₂ → CO₂ + H₂O
C₃H₈ + 5O₂ → 3CO₂ + 4H₂O
Balance: Al₂(SO₄)₃ + Ca(OH)₂ → Al(OH)₃ + CaSO₄
Al₂(SO₄)₃ + 3Ca(OH)₂ → 2Al(OH)₃ + 3CaSO₄
In a reaction, atoms are rearranged into new substances. Explain how this supports conservation of mass.
The reaction changes how atoms are bonded, but the atoms remain. Because the same atoms are present before and after, total mass is conserved
A student writes 2H₂ + O₂ → H₂O₂ and says it is balanced because the atom counts match. What is wrong with this approach?
The student changed the product formula. The equation must keep the given formulas; balance it using coefficients only. The correctly balanced equation is 2H₂ + O₂ → 2H₂O.
Balance: Al + O₂ → Al₂O₃
4Al + 3O₂ → 2Al₂O₃
Balance: C₂H₆ + O₂ → CO₂ + H₂O
2C₂H₆ + 7O₂ → 4CO₂ + 6H₂O
Balance: Fe₂(SO₄)₃ + KOH → K₂SO₄ + Fe(OH)₃
Fe₂(SO₄)₃ + 6KOH → 3K₂SO₄ + 2Fe(OH)₃