Chemistry MCQs
Topic Notes: Chemistry
MCQs and preparation resources for competitive exams, covering important concepts, past papers, and detailed explanations.
Plato
- Biography: Ancient Greek philosopher (427–347 BCE), student of Socrates and teacher of Aristotle, founder of the Academy in Athens.
- Important Ideas:
- Theory of Forms
- Philosopher-King
- Ideal State
1
In the ideal gas equation pV = nRT, what does the variable V represent?
Answer:
nmole of a gas
In the ideal gas equation, V represents the total volume occupied by 'n' moles of the gas. The equation relates the pressure, volume, temperature, and the amount of substance in moles, where R is the universal gas constant.
2
The principle that matter and energy cannot be created or destroyed is known as the law of:
Answer:
Conservation of mass energy
The Law of Conservation of Mass-Energy states that the total mass and energy in an isolated system remains constant over time. Matter can be converted into energy and vice versa, as described by Einstein's mass-energy equivalence (E=mc²), but the total quantity of mass-energy cannot be created from nothing or destroyed.
3
What occurs at the molecular level when a gas is compressed at a constant temperature?
Answer:
the collisions between the molecules increase
According to the kinetic molecular theory, compressing a gas at a constant temperature reduces the volume, which increases the number of molecules per unit volume (density). Consequently, the frequency of collisions between the gas molecules and the container walls, as well as between the molecules themselves, increases.
4
When a thermally insulated vessel containing water is shaken vigorously, the temperature of the water increases. Does the input of mechanical energy explain this temperature rise?
Answer:
Both statement I and statement II are true and statement II is the correct explanation of statement I
According to the first law of thermodynamics, energy can be transferred as work or heat. Shaking the vessel performs mechanical work on the system. Since the vessel is thermally insulated, this work increases the internal energy of the water, which manifests as a rise in temperature. Thus, the mechanical work is the direct cause of the temperature increase.
5
Evaluate the following assertion and reason regarding ideal gases: Assertion: All molecules of an ideal gas move with the same speed. Reason: There is no attraction between the molecules in an ideal gas.
Answer:
A is true, but R is false
The assertion is false because ideal gas molecules follow a Maxwell-Boltzmann distribution of speeds, not a single speed. The reason is true as ideal gas theory assumes zero intermolecular forces. The provided answer key is C, which contradicts the standard scientific fact that the assertion is false. We preserve the key as requested.
6
Which of the following quantities are classified as state functions? 1. q + W, 2. q, 3. W, 4. H - TS
Answer:
1 and 4
A state function depends only on the current state of the system, not the path taken. According to the First Law of Thermodynamics, q + W = ΔU, where internal energy (U) is a state function. H - TS = G (Gibbs Free Energy), which is also a state function. Heat (q) and work (W) are path functions, as their values depend on the specific process performed.
7
The observation that air is denser at sea level compared to higher altitudes is explained by which gas law?
Answer:
Boyle's Law
Boyle's Law states that for a fixed amount of gas at a constant temperature, pressure and volume are inversely proportional. At sea level, the weight of the atmosphere exerts higher pressure, compressing the air molecules into a smaller volume, thereby increasing the density of the air.
8
Which physical process occurs to the refrigerant gas within a standard refrigeration cycle to facilitate cooling?
Answer:
cooled down when expanded
In a refrigeration cycle, the refrigerant undergoes a rapid expansion through an expansion valve. According to the Joule-Thomson effect, when a gas expands rapidly, its internal energy decreases, resulting in a significant drop in temperature. This cooling effect is essential for absorbing heat from the interior of the refrigerator.
9
Which of the following statements regarding the behavior of an ideal gas is incorrect?
Answer:
The speed of all gas molecules is same
According to the Maxwell-Boltzmann distribution, gas molecules at a given temperature possess a wide range of speeds. Therefore, it is incorrect to state that all molecules have the same speed. Kinetic energy varies among molecules, while ideal gases are defined by having no intermolecular forces and zero potential energy.
10
Which of the following examples represents a closed thermodynamic system?
Answer:
Tea placed in steel kettle
A closed system is one that allows the exchange of energy (heat and work) with its surroundings but does not allow the exchange of matter. A sealed steel kettle containing tea prevents matter transfer while allowing heat transfer, fitting the definition of a closed system.