Question

when kim added 30 g of hydrochloric acid (hcl) to 65 g of zn, bubbles appeared and a white precipitate, zinc chloride (zncl₂) was produced. she found that the mass of the zinc chloride was 93 g. since the law of conservation of matter stated that matter can’t be created or destroyed, she was puzzled about the difference in mass. what is the best explanation for the missing 2 g?
a 2 g of zinc disappeared
b 2 g of hydrogen gas were produced
c kim didn’t correctly mass the reactants
d kim didn’t collect all of the precipitate

Explanation:

The reaction between zinc (Zn) and hydrochloric acid (HCl) is \( \text{Zn} + 2\text{HCl}
ightarrow \text{ZnCl}_2 + \text{H}_2 \uparrow \). The total mass of reactants is \( 30\,\text{g (HCl)} + 65\,\text{g (Zn)} = 95\,\text{g} \). The mass of \( \text{ZnCl}_2 \) is \( 93\,\text{g} \). According to the Law of Conservation of Mass, the missing mass should be accounted for by the gas produced (hydrogen gas, \( \text{H}_2 \)). Option A is incorrect as matter can't disappear. Option C and D are less likely as the problem implies a chemical explanation, and the gas (which is not a precipitate) was not measured, so the missing \( 2\,\text{g} \) is the hydrogen gas produced.

Answer:

B. 2 g of hydrogen gas were produced