In open flasks, which of these reactions will show a decrease in mass during the reaction? Select all that apply: (1) 2 H₂O₂ (aq) ⟶ 2 H₂O (l) + O₂ (g) (2) ZnCO₃ (s) + 2 HCl (aq) ⟶ ZnCl₂ (aq) + CO₂ (g) + H₂O (l) (3) Mg (s) + 2 HCl (aq) ⟶ MgCl₂ (aq) + H₂ (g) (4) BaCl₂ (aq) + Na₂SO₄ (aq) ⟶ 2 NaCl (aq) + BaSO₄ (s)

What decides whether the mass drops in an open flask?

In a closed container, total mass stays constant. But in an open flask, any gaseous product can leave the flask, so the measured mass of the flask and its contents can decrease.

Checking each reaction for a gas product

1. $$2H_2O_2(aq) \rightarrow 2H_2O(l) + O_2(g)$$ Produces $O_2(g)$, so gas escapes. Mass decreases.

2. $$ZnCO_3(s) + 2HCl(aq) \rightarrow ZnCl_2(aq) + CO_2(g) + H_2O(l)$$ Produces $CO_2(g)$, so gas escapes. Mass decreases.

3. $$Mg(s) + 2HCl(aq) \rightarrow MgCl_2(aq) + H_2(g)$$ Produces $H_2(g)$, so gas escapes. Mass decreases.

4. $$BaCl_2(aq) + Na_2SO_4(aq) \rightarrow 2NaCl(aq) + BaSO_4(s)$$ No gas forms, a solid $BaSO_4(s)$ precipitate stays in the flask. No mass decrease (aside from minor effects like evaporation, which are not part of the reaction).

Final selection

Choose reactions (1), (2), and (3).