Which equation obeys the law of conservation of mass? h2(g) o2(g) → h2o(g) h2(g) o2(g) → h2o(g) 4he(g) 2h2(g) o2(g) → 2h2o(g) h2(g) → h2o(g) h2(g) o2(g) → 2h2o(g)