A scientist conducts an experiment by heating $50\mathrm{g}$ of mercury(II) oxide in a closed container. After heating, it decomposes into mercury and oxygen gas. What will be the total mass of the products?

Appeared in: RRB Nsg. Superintendent -29 April-2025 (shift-2nd)

Explanation

This question tests the Law of Conservation of Mass, which states that in a closed system, mass is neither created nor destroyed during a chemical reaction.
The reaction takes place in a 'closed container', which is the key piece of information indicating that it is a closed system.
Because no matter can escape, the total mass of the products (mercury and oxygen gas) must be exactly equal to the starting mass of the reactant (mercury(II) oxide).
Therefore, if the experiment starts with 50 g of reactant, it must end with a total of 50 g of products.

Option A: The law of conservation of mass applies to the total mass of the system. While the individual masses of mercury and oxygen are specific values determined by stoichiometry, they are not needed to find the total combined mass, which must equal the initial mass.
Option B: Mass would only decrease if a product could escape the system. The term 'closed container' explicitly rules out this possibility.
Option D: Mass cannot be created from nothing in a chemical reaction. This option violates the Law of Conservation of Mass.

Visual 1: Diagram: A simple diagram showing a weighing balance. On one side, a closed flask containing a red powder (mercury(II) oxide) reads 50 g. On the other side, the same closed flask now containing a silver liquid (mercury) and a gas (oxygen) also reads 50 g. This visually reinforces that mass is conserved in a closed system.

Clinical Relevance

Nursing practice connection: Safe nursing care depends on performing Law of Conservation of Mass in the correct sequence, documenting the action clearly, and monitoring for the expected response.
The chemical in the question, mercury, is highly toxic and was once common in medical equipment like thermometers and sphygmomanometers. Nurses must be aware of the dangers and proper handling/disposal protocols for this hazardous material.
Mercury exposure is a significant health risk, acting as a neurotoxin. Chronic exposure can cause tremors and cognitive issues ('mad as a hatter'), while acute exposure can damage the lungs and kidneys.
Patient safety requires phasing out mercury-based devices and ensuring staff are trained on proper mercury spill management to prevent toxic exposure in clinical settings.

How to Approach the Question

First, identify the core scientific principle being tested. The question involves a chemical reaction and asks about the relationship between the mass of reactants and products.
Scan the stem for keywords. The most critical phrase is 'closed container'. This tells you that the system is isolated, and no matter can enter or leave.
Recall the Law of Conservation of Mass. This fundamental law states that for any closed system, the total mass must remain constant over time.
Apply the law to the given values. The initial mass of the reactant is 50 g. Since the system is closed, the total final mass of all products must also be 50 g.
Evaluate the options based on this principle. The option stating the mass is conserved ('Exactly 50 g') is the correct one. Eliminate options that suggest mass is lost, gained, or cannot be determined in a closed system.

Concept Tested & Keywords

Concept Tested: Law of Conservation of Mass
Stem keywords: heating, mercury(II) oxide, closed container, decomposes, total mass
Lead-in keywords: What will be
Clinical cues: closed container (indicates a closed system with no mass loss or gain)

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Practise the full RRB Nsg. Superintendent -29 April-2025 (shift-2nd)

Attempt every question from this paper in a timed mock, then review the full solution for each one.