2020

121. Which one of the following aqueous solutions will be neutral ?

Which one of the following aqueous solutions will be neutral ?

[amp_mcq option1=”NH₄Cl” option2=”NaCl” option3=”KCN” option4=”NaHSO₄” correct=”option2″]

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An aqueous solution is neutral if the salt dissolved in water is formed from the reaction of a strong acid and a strong base.
A) NH₄Cl is formed from a weak base (NH₄OH) and a strong acid (HCl). The solution will be acidic due to the hydrolysis of NH₄⁺ ions.
B) NaCl is formed from a strong base (NaOH) and a strong acid (HCl). The solution will be neutral as neither Na⁺ nor Cl⁻ ions undergo significant hydrolysis.
C) KCN is formed from a strong base (KOH) and a weak acid (HCN). The solution will be basic due to the hydrolysis of CN⁻ ions.
D) NaHSO₄ is sodium bisulfate. It is formed from a strong base (NaOH) and sulfuric acid (H₂SO₄), which is a strong acid. However, HSO₄⁻ is the conjugate base of the strong acid H₂SO₄, but it is also an acid itself that can donate a proton (HSO₄⁻ ⇌ H⁺ + SO₄²⁻). This makes the solution acidic.
Therefore, only NaCl forms a neutral aqueous solution.
The neutrality of an aqueous salt solution depends on the strength of the parent acid and base from which the salt is derived. Salts of strong acid and strong base yield neutral solutions. Salts of strong acid and weak base yield acidic solutions. Salts of weak acid and strong base yield basic solutions. Salts of weak acid and weak base yield solutions whose pH depends on the relative strengths of the weak acid and weak base.
Hydrolysis is the reaction of an ion with water. Cations of weak bases undergo acidic hydrolysis, releasing H⁺ ions. Anions of weak acids undergo basic hydrolysis, releasing OH⁻ ions. Cations of strong bases and anions of strong acids do not undergo significant hydrolysis.

122. Manganese is extracted from Manganese dioxide by reaction with Alumini

Manganese is extracted from Manganese dioxide by reaction with Aluminium as described by the following unbalanced chemical equation :
MnO₂(s) + Al (s) → Mn (l) + Al₂O₃ (s)
The number of moles of Al (s) required to form one mole of Mn from its oxide is

[amp_mcq option1=”1″ option2=”0.75″ option3=”1.33″ option4=”2″ correct=”option3″]

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The unbalanced chemical equation is MnO₂(s) + Al (s) → Mn (l) + Al₂O₃ (s). To determine the moles of Al required to form one mole of Mn, we first need to balance the equation.
Balancing the oxygen atoms (2 on the left, 3 on the right), we find the least common multiple is 6. Multiply MnO₂ by 3 and Al₂O₃ by 2:
3 MnO₂(s) + Al (s) → Mn (l) + 2 Al₂O₃ (s)
Now, balance the aluminium atoms (1 on the left, 4 on the right). Multiply Al by 4:
3 MnO₂(s) + 4 Al (s) → Mn (l) + 2 Al₂O₃ (s)
Finally, balance the manganese atoms (3 on the left, 1 on the right). Multiply Mn by 3:
3 MnO₂(s) + 4 Al (s) → 3 Mn (l) + 2 Al₂O₃ (s)
The balanced equation is 3 MnO₂(s) + 4 Al (s) → 3 Mn (l) + 2 Al₂O₃ (s).
According to the balanced equation, 4 moles of Al react to produce 3 moles of Mn. To produce 1 mole of Mn, the number of moles of Al required is (4 moles Al / 3 moles Mn) * 1 mole Mn = 4/3 moles Al.
The question requires balancing the given chemical equation and then using stoichiometry to find the mole ratio between reactants and products. The ratio of Al to Mn in the balanced equation is 4:3.
This reaction is a type of redox reaction, specifically a thermite reaction where a metal oxide is reduced by a more reactive metal (Aluminium). Aluminium is a strong reducing agent. The value 4/3 is approximately 1.333…

123. How many moles of water would be produced by the complete combustion o

How many moles of water would be produced by the complete combustion of one mole of natural gas, CH₄, in excess of oxygen ?

[amp_mcq option1=”1″ option2=”2″ option3=”3″ option4=”4″ correct=”option2″]

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The complete combustion of natural gas, which is primarily methane (CH₄), in excess oxygen is represented by the balanced chemical equation:
CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(g or l)
The coefficients in the balanced equation represent the relative number of moles of reactants and products. According to the equation, one mole of methane (CH₄) reacts with two moles of oxygen (O₂) to produce one mole of carbon dioxide (CO₂) and two moles of water (H₂O).
The stoichiometry of a balanced chemical equation gives the mole ratios of reactants and products. For the combustion of methane, 1 mole of CH₄ produces 2 moles of H₂O.
Combustion is a rapid reaction between a substance with an oxidant, usually oxygen, to produce heat and light. Complete combustion occurs when there is sufficient oxygen, yielding carbon dioxide and water as products for hydrocarbons like methane. Incomplete combustion occurs with insufficient oxygen, producing carbon monoxide and/or carbon. The question specifies “complete combustion in excess of oxygen”, ensuring the reaction proceeds as shown in the balanced equation.

124. An element has an atomic number of 16. What is the principal quantum n

An element has an atomic number of 16. What is the principal quantum number (n) of its outermost electrons ?

[amp_mcq option1=”1″ option2=”2″ option3=”3″ option4=”4″ correct=”option3″]

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An element with atomic number 16 is Sulfur (S). To find the principal quantum number (n) of its outermost electrons, we need to write its electron configuration. The atomic number represents the number of protons and, in a neutral atom, the number of electrons. So, Sulfur has 16 electrons. The electron configuration is filled according to the Aufbau principle, Hund’s rule, and the Pauli exclusion principle.
Filling order: 1s, 2s, 2p, 3s, 3p, 4s, 3d, …
Electrons:
1s² (2 electrons)
2s² (2 electrons)
2p⁶ (6 electrons) – total 2+2+6 = 10 electrons
3s² (2 electrons) – total 10+2 = 12 electrons
3p⁴ (4 electrons) – total 12+4 = 16 electrons
The electron configuration is 1s² 2s² 2p⁶ 3s² 3p⁴. The outermost electrons are those in the highest principal energy level, which is n=3 in this case (3s and 3p orbitals).
The principal quantum number (n) of the outermost electrons corresponds to the highest energy level occupied by electrons in the atom’s electron configuration. For Sulfur (atomic number 16), the configuration is 1s² 2s² 2p⁶ 3s² 3p⁴, so the outermost electrons are in the n=3 level.
The outer electron shell is often referred to as the valence shell. The principal quantum number ‘n’ defines the energy level and size of the electron shell. n=1 is the first shell, n=2 is the second shell, n=3 is the third shell, and so on.

125. Gases can be liquefied by

Gases can be liquefied by

[amp_mcq option1=”reducing pressure and temperature.” option2=”applying pressure and reducing temperature.” option3=”reducing pressure and raising temperature.” option4=”applying pressure and raising temperature.” correct=”option2″]

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Gases can be liquefied by increasing the intermolecular forces or reducing the kinetic energy of the gas particles so that they can come closer together and form a liquid state. Applying pressure forces the particles closer, increasing intermolecular interactions. Reducing temperature decreases the kinetic energy of the particles, making it easier for them to form liquid bonds. Liquefaction occurs when the temperature is at or below the critical temperature and sufficient pressure is applied. The most effective way is typically applying pressure and reducing temperature.
Liquefaction of gases requires increasing pressure and/or decreasing temperature to bring molecules closer and reduce their kinetic energy.
For every gas, there is a critical temperature above which it cannot be liquefied by pressure alone. Below the critical temperature, increasing pressure can cause liquefaction. Reducing the temperature makes liquefaction easier at lower pressures. Therefore, applying pressure and reducing temperature together is the standard method for liquefying gases.

126. In an atomic gas, the motion of particles (atoms) is governed by the c

In an atomic gas, the motion of particles (atoms) is governed by the collisions. If the gas is ionized, then the motion of created particles may be mainly governed by

[amp_mcq option1=”gravitational force.” option2=”collisions.” option3=”scattering of particles.” option4=”electromagnetic force between the particles.” correct=”option4″]

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In an atomic gas, the atoms are electrically neutral, and their motion is primarily governed by collisions between them. When the gas is ionized, atoms lose or gain electrons, becoming charged particles (ions and free electrons). These charged particles exert strong electrostatic (electromagnetic) forces on each other over relatively long distances compared to the short-range forces involved in neutral collisions. Therefore, the motion of particles in an ionized gas (plasma) is mainly governed by the long-range electromagnetic forces between these charged particles, rather than just collisions.
In a neutral atomic gas, particle motion is dominated by collisions. In an ionized gas (plasma) containing charged particles, the motion is dominated by long-range electromagnetic forces between these charges.
An ionized gas is also known as plasma, which is often considered the fourth state of matter. The collective behavior of charged particles under the influence of electromagnetic fields is a key characteristic of plasma physics. While collisions still occur in plasma, their influence on overall motion is often less dominant than the electromagnetic forces, especially in hot, tenuous plasmas.

127. The transfer of thermal energy carries which of the following

The transfer of thermal energy carries which of the following phenomena?

[amp_mcq option1=”Conduction and convection only” option2=”Only conduction” option3=”Conduction, convection and radiation” option4=”Only radiation” correct=”option3″]

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Thermal energy, or heat, can be transferred from one place to another through three fundamental mechanisms: conduction, convection, and radiation. Conduction is the transfer of heat through direct contact of particles, primarily in solids. Convection is the transfer of heat through the movement of fluids (liquids or gases). Radiation is the transfer of heat through electromagnetic waves, which does not require a medium and can occur through a vacuum.
The three main modes of thermal energy transfer are conduction, convection, and radiation.
Examples: Conduction transfers heat through a metal rod when one end is heated. Convection transfers heat in boiling water or rising hot air. Radiation transfers heat from the sun to the Earth, or from a fire to your hands. All three phenomena contribute to the transfer of thermal energy in different situations.

128. An electric refrigerator rated 400 W operates 10 hours/day. What is th

An electric refrigerator rated 400 W operates 10 hours/day. What is the cost of the energy to operate it for 30 days at ₹ 3.00 per kWh?

[amp_mcq option1=”₹ 360″ option2=”₹ 3,600″ option3=”₹ 36″ option4=”₹ 400″ correct=”option1″]

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The power rating of the refrigerator is 400 W, which is equal to 0.4 kW (since 1 kW = 1000 W). The refrigerator operates for 10 hours per day. Over 30 days, the total operation time is 10 hours/day * 30 days = 300 hours. The total energy consumed is the power multiplied by the time: Energy (kWh) = Power (kW) * Time (hours) = 0.4 kW * 300 hours = 120 kWh. The cost of energy is ₹ 3.00 per kWh. Total cost = Energy consumed * Cost per kWh = 120 kWh * ₹ 3.00/kWh = ₹ 360.
Energy consumed is calculated as Power (in kW) multiplied by Time (in hours). The total cost is the energy consumed multiplied by the rate per unit of energy (kWh).
The unit of energy used for billing is typically the kilowatt-hour (kWh), often called a ‘unit’ of electricity. Power in watts needs to be converted to kilowatts before calculating energy in kWh if time is in hours.

129. An object is kept at infinity from the position of a concave (spherica

An object is kept at infinity from the position of a concave (spherical) mirror. Which one is *not* true about the image of the object?

[amp_mcq option1=”Position of image is at the focus of the mirror” option2=”Size of image is the same as that of the object” option3=”Image is real” option4=”Image is inverted” correct=”option2″]

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When an object is placed at infinity from a concave spherical mirror, the light rays from the object are considered to be parallel to the principal axis. After reflection from the concave mirror, these parallel rays converge at the principal focus (F) of the mirror. The image formed at the focus is real, inverted (relative to the infinitely distant object), and highly diminished (essentially a point image).
For an object at infinity from a concave mirror, the image is formed at the focus, is real, inverted, and highly diminished (point-sized).
Option A states the position of the image is at the focus, which is true. Option C states the image is real, which is true for converging rays formed on the same side as the object. Option D states the image is inverted, which is true for real images formed by a concave mirror (even though a point image doesn’t visually appear inverted). Option B states the size of the image is the same as that of the object, which is false; the image is highly diminished. Therefore, the statement that is *not* true is B.

130. Tyndall effect appears due to which one of the following properties of

Tyndall effect appears due to which one of the following properties of light?

[amp_mcq option1=”Reflection of light” option2=”Diffraction of light” option3=”Polarization of light” option4=”Scattering of light” correct=”option4″]

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The Tyndall effect is the phenomenon where the path of a beam of light becomes visible as it passes through a colloidal dispersion or a fine suspension. This effect occurs because the larger particles in the colloid or suspension scatter the light in all directions when it strikes them.
Tyndall effect is the scattering of light by particles in a colloid or a very fine suspension.
Reflection occurs when light bounces off a surface. Diffraction is the bending of light waves as they pass around the edge of an obstacle or through a narrow slit. Polarization is the restriction of the vibration of light waves to a single plane. Scattering is the process by which light is deflected in various directions as it interacts with a medium or particles within it, which is the principle behind the Tyndall effect.
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