Question

A group of students wants to observe the reaction between magnesium metal and hydrochloric acid. They place a 1.2-g strip of the metal into a test tube containing 4.7 g of aqueous hydrochloric acid. The students observe bubbles forming on the surface of the metal, and the solution turns white. After all of the metal disappears, they use a triple-beam balance to determine the mass of the products. The resulting mass is less than the mass of the reactants, which leads them to believe they did something incorrectly during their experiment. Examine the equation below, then determine which of the following steps the students could have taken to prove that mass was conserved during the reaction. Mg(s) + HCl(aq) ⇒ H2(g) + MgCl2(aq) A The students could recalibrate the triple-beam balance before finding the mass of the solution again. B The students could hold a glowing splint over the top of the test tube to check for the presence of hydrogen gas. C The students could have collected the hydrogen gas formed during the reaction, found its mass, and added that amount to the mass of the solution. D The students could boil the solution that formed to remove excess water, weigh the solid magnesium chloride, then compare that mass to the mass of the reactants.

Answer 1

Answer:

The students could hold a glowing splint over the top of the test tube to check for the presence of hydrogen gas and the students could have collected the hydrogen gas formed during the reaction, found its mass, and added that amount to the mass of the solution

Explanation:

The mass of any reaction, assuming 100 % yield, should be conserved according to the law of mass conservation: mass cannot be created nor destroyed.

In the experiment described, the essential reason why the mass of the products is lower than the mass of the reactants is simply the evolution of a gas.

Having hydrogen formed in this reaction, it escapes the flask into the surroundings and we actually have a loss of hydrogen in our system, as it's not in the solution anymore (hydrogen gas is extremely weakly soluble in water).

This means, in order to prove that the experiment was actually carried out successfully, we'd firstly need to prove the presence of hydrogen using the glowing split test. Then, upon verifying the presence of hydrogen gas, it should be collected and its mass found, so that it could be added to the mass of magnesium chloride solution and we would then obtain the total mass of the products formed.