Question

Which chemical equation supports the law of conservation of mass?


4H2O(l) à 4H2(g) + 2O2(g)


2CH4(g) + 2O2(g) à CO2(g) + 4H2O(g)

Answer 1

The first equation supports the law of conservation of mass; the second does not.

1) 4 H2O(l) → 4 H2(g) + 2 O2(g)
- Left: H = 4×2 = 8, O = 4×1 = 4
- Right: H = 4×2 = 8, O = 2×2 = 4
Atoms (and thus mass) are conserved.

2) 2 CH4(g) + 2 O2(g) → CO2(g) + 4 H2O(g)
- Left: C = 2, H = 8, O = 4
- Right: C = 1, H = 8, O = 2 + 4 = 6
Carbons and oxygens are not balanced, so this equation does not conserve mass as written.

A correct balanced combustion of methane is:
CH4 + 2 O2 → CO2 + 2 H2O
(or multiplied through: 2 CH4 + 4 O2 → 2 CO2 + 4 H2O).