Chemical Sciences MCQ Quiz - Objective Question with Answer for Chemical Sciences - Download Free PDF

CONCEPT:

Thiocyanogen (SCN)₂

• Thiocyanogen ((SCN)₂) is a compound that is composed of two thiocyanate (SCN) groups linked together.
• It is a highly reactive compound and is unstable at higher temperatures due to its tendency to decompose.
• To maintain its stability, the conditions must favor minimal thermal agitation, which is achieved at low temperatures.

EXPLANATION:

• The stability of thiocyanogen ((SCN)₂) depends on the temperature:
  • At high or very high temperatures, the compound decomposes rapidly due to increased molecular motion and energy.
  • At room temperature, it is still prone to decomposition, though at a slower rate compared to higher temperatures.
  • At low temperatures, the thermal energy is reduced, preventing rapid decomposition and thus stabilizing the compound.

Therefore, thiocyanogen ((SCN)₂) is stable only at low temperatures.

CONCEPT:

δ (Delta) Bonds in Transition Metal Compounds

• δ-bonds are formed by the sidewise overlap of two d-orbitals that have four lobes oriented between the axes.
• These bonds are rarer and generally occur in compounds with metal–metal multiple bonding, especially in transition metal dimers.
• To form a δ-bond, the orbitals must overlap sideways in such a way that the nodal planes pass through the internuclear axis.

EXPLANATION:

• d_xy: Lobes lie between x and y axes → suitable for δ-bond 
• d_yz: Lobes between y and z axes → not ideal for δ-bond in typical M–M direction 
• d_z²: Lobes along z-axis → forms σ-bonds, not δ 
• d_x²−y²: Lobes along x and y axes → forms σ-bonds, not δ 

Therefore, the orbital that forms a δ-bond is d_xy

CONCEPT:

Activity Coefficient of Electrolytes

• The activity coefficient (γ±) is a measure of the deviation of an electrolyte's behavior from ideality in solution.
• For strong electrolytes like BaCl₂, the activity coefficient is influenced by the ionic strength of the solution and the interactions between ions.
• The Debye-Hückel theory provides a way to approximate the activity coefficients for electrolytes in dilute solutions.

EXPLANATION:

• Barium chloride (BaCl₂) dissociates into ions as:

  BaCl₂(aq) → Ba²⁺(aq) + 2Cl^-(aq)

• The activity coefficient (γ±) is calculated for the overall ionic strength of the dissociated ions.
• 4γ ±³ m³ is correct because BaCl₂ dissociates into 1 Ba²⁺ ion and 2 Cl^- ions (total 3 particles), and the coefficient aligns with this dissociation.

Therefore, the correct answer is 4γ ±³ m³.

CONCEPT:

Nonactin as an Ionophore

• Nonactin is a type of ionophore, which is a chemical compound that facilitates the transport of ions across lipid membranes.
• It is known for its selective binding to specific metal ions, particularly potassium ions (K⁺).
• The structure of Nonactin contains a cyclic arrangement that forms a cavity suitable for binding K⁺ ions due to their size and charge.
• This selective binding is due to the size compatibility of the K⁺ ion with the cavity created by the cyclic Nonactin molecule, as well as favorable interactions between the ion and the oxygen atoms in Nonactin.

EXPLANATION:

• Nonactin exhibits high specificity for K⁺ ions over other metal ions like Na⁺, Mg²⁺, and Ca²⁺.
• This specificity is crucial for its biological role, especially in processes like transporting potassium ions across membranes in cells.
• Other ions like Na⁺ are smaller and do not fit as well into the cavity of Nonactin, while larger divalent ions like Mg²⁺ and Ca²⁺ are less compatible with the binding site due to their charge and hydration characteristics.
• Therefore, Nonactin is most effective in binding and transporting K⁺ ions, making option 1 the correct answer.

Therefore, the compound Nonactin specifically binds to potassium ions (K⁺).

CONCEPT:

Effective Nuclear Charge (Z_eff)

• Effective nuclear charge (Z_eff) is the net positive charge experienced by an electron in a multi-electron atom.
• Z_eff accounts for the shielding or screening effect of inner electrons that reduce the full attractive force of the nucleus on the outer electrons.
• The shielding constant (σ) quantifies the extent to which inner electrons block the nuclear charge from outer electrons.
• The relationship between the atomic number (Z), effective nuclear charge (Z_eff), and shielding constant (σ) is given by the formula:

  Z_eff = Z - σ

EXPLANATION:

• The atomic number (Z) represents the total number of protons in the nucleus of an atom.
• The shielding constant (σ) represents the reduction in nuclear attraction due to the repulsion caused by inner electrons.
• The effective nuclear charge (Z_eff) is the actual positive charge "felt" by an electron after accounting for the shielding effect.
• Using the relationship Z_eff = Z - σ:
  • If Z is the total nuclear charge and σ is the shielding constant, subtracting σ from Z gives the net effective nuclear charge (Z_eff).
• Z_eff = Z - σ (Correct, as it accurately represents the relationship).

​Therefore, the correct answer is Z_eff = Z - σ.

The correct answer is Uracil.

Key Points

• In DNA, Adenine, Guanine, Thymine, and Cytosine are present.
• In RNA Adenine, Guanine, Cytosine, and Uracil are present.
• Uracil
  • It is one of the four nucleobases in nucleic acid RNA.
  • In DNA, Uracil nucleobases are replaced by thymine.
  • Its formula is C₄H₄N₂O_2.

_{Additional Information}

•  Guanine
  • It is one of the four main nucleobases found in the nucleic acids DNA and RNA.
  • It is used to make one of the building blocks of DNA and RNA.
• Adenine
  • It is also one of the four main nucleobases found in the nucleic acids DNA and RNA.
  • It is part of many substances in the body that give energy to cells.
• Cytosine
  • It is pyrimidine and one of the nitrogenous bases found in RNA and DNA acid for making genetic code.
  • It Plays an essential role in forming base pairs by bonding with guanine.

• The properties of the ionic compounds include:-
• Solid structure and hard surface
• High Melting and boiling points
• Solubility in water
• The ionic compounds have high melting and boiling points due to the strong forces of attraction present between the particles of the compound.
• The ionic bonding grants strength to the structure.
• Thus, Low melting and boiling points are not the property of an ionic compound.

Key Points

• The s-block element that is a silvery- metal and is used in an alloy with copper or nickel to make various equipment is Beryllium.
• Beryllium is a lightweight and strong metal that is widely used in aerospace, defense, and nuclear industries.
• It has a high melting point, good thermal conductivity, and is also a good electrical conductor.
• Atomic number of Beryllium is - 4.

Additional Information

• Rubidium is a soft, silvery- metal that is highly reactive.
  • It is used in atomic clocks and as a catalyst in certain chemical reactions.
• Francium is a highly radioactive and unstable element.
  • It is extremely rare and has no practical applications.
• Caesium is a soft, silvery-golden metal that is also highly reactive.
  • It is used in atomic clocks, drilling fluids, and in the treatment of cancer.

Concept:

Median

Case 1: If number of observation (n) is even

\({\rm{Median\;}} = {\rm{\;}}\frac{{{\rm{value\;of\;}}{{\left( {\frac{{\rm{n}}}{2}} \right)}^{{\rm{th}}}}{\rm{\;observation\;}} + {\rm{\;\;value\;of\;}}{{\left( {\frac{{\rm{n}}}{2}{\rm{\;}} + 1} \right)}^{{\rm{th}}}}{\rm{\;observation}}}}{2}\)

Case 2: If number of observation (n) is odd

\({\rm{Median\;}} = {\rm{value\;of\;}}{\left( {\frac{{{\rm{n}} + 1}}{2}} \right)^{{\rm{th}}}}{\rm{\;observation}}\)

Calculation:

Arrange the observations in the ascending order are

1, 2, 3, 6, 8, 9

Here, n = 6 = even.

So, 3^rd and 4^th observation are 3 and 6

∴\({\rm{Median}} = {\rm{\;}}\frac{{3 + 6}}{2} = 4.5\)

The correct answer is NH₃ and HCl

Key Points

• Dalton's law is applicable only to a mixture of non-reacting gases.
• Ammonia (NH₃) reacts with hydrochloric acid at normal temperature and gives ammonium chloride(NH₄Cl).
• NH₃ + HCl    →    NH₄Cl
• So, this gaseous mixture does not follow Dalton's law.

Additional Information

Dalton's law:

• Dalton's law states that in a mixture of non-reacting gases, the total pressure exerted is equal to the sum of the partial pressures of the individual gases.
• This empirical law was observed by John Dalton in 1801.
• It was published in 1802.
• It is also called Dalton's law of partial pressures.
• Dalton's law is related to the ideal gas laws.
• Dalton's law is not strictly followed by real gases, with the deviation increasing with pressure.

Important Points

• Helium is a noble gas that will not react with hydrogen gas, so this gaseous mixture follows Dalton’s law.
• Nitrogen gas is not reactive in normal temperatures because the breakdown of nitrogen into atomic form is a highly endothermic process.
• Nitrogen and oxygen gas are not found in their atomic form in normal temperatures, so these gases will not react with each other. 

Explanation:- 

• Allotropes of carbon like Diamond, Graphite, and Fullerene show crystalline structures.
• Fullerene has infinite lattices of Diamond and Graphite.
• It is formed by nanotubes forming the discrete molecular structure.

Additional Information 

• Fullerene molecule consists of carbon atoms connected by single and double bonds.
• Common structures are C₆₀ and C_70.

The correct answer is S > Se > O

Concept:

• The amount of energy released when an extra electron is added to a neutral atom to form a ​negatively charged ion is called electron affinity.
• Since energy is released, it is represented with negative sign.

​               \(A+ e^- \rightarrow A^- \)    .... \(\Delta _{aff}H = -ive \)

• Atoms with high electron affinity have higher tendency to accept the electron.
• Electron affinity depends on Nuclear Charge, Atomic size as well as the electron distribution in orbitals.

Explanation:

• Down the group, new shell is added. The size of atom increases and therefore, the attraction between nucleus and outermost electron decreases which further decreases the electron affinity.
• Hence  K > Li > Na is incorrect as K is the largest among three elements
• The small atoms cannot accommodate incoming electron and undergo inter-electronic repulsions. Thus, First element of the group will have least electron affinity for one-electron gain.

Thus, F > Cl > Br and P > N > As are incorrect as F and N being smallest element should have least electron affinity.

Conclusion:

Hence, the correct  order of electron affinity for one-electron gain is: S > Se > O

13, 8, 15, 14, 17, 9, 14, 16, 13, 17, 14, 15, 16, 15, 14

First of all, we arrange the data in ascending order, then

8, 9, 13, 13, 14, 14, 14, 14, 15, 15, 15, 16, 16, 17, 17

Total number (n) = 15

As we know,

Median = [(n + 1)/2]^th = [(15 + 1)/2]^th = [16/2]^th = 8^th number

8^th number is 14

So, x = 14

If 8 is replaced by 18 then,

13, 18, 15, 14, 17, 9, 14, 16, 13, 17, 14, 15, 16, 15, 14

Again, we arrange the data in ascending order, then

9, 13, 13, 14, 14, 14, 14, 15, 15, 15, 16, 16, 17, 17, 18

Total number (n) = 15

Median = [(n + 1)/2]^th = [(15 + 1)/2]^th = [16/2]^th = 8^th number

8^th number is 15

So, y = 14

x + y

⇒ 14 + 15

Concept:

Explanation:

• Option (1) is incorrect because ricin is a toxin obtained from Ricinus plant.
• Vinblastin and curcumin are drugs.
• Morphine and codeine are alkaloids.
• Abrin is also a toxin obtained by the plant Abrus.
• Concanavalin A is a lectin.​

The correct answer is - 3) Protein

Concept:

Fat -

• Fats are a type of lipid consisting of triesters of glycerol and fatty acids or triglycerides. 

Carbohydrates -

• Carbohydrates are defined as “optically active polyhydroxy aldehydes or ketones or the compounds which produce units of such type on hydrolysis”. 

Protein - 

• A protein is a naturally occurring, extremely complex substance that consists of amino acid residues joined by peptide bonds.

Nucleic Acid - 

• Nucleic acids are polynucleotides that are, long chainlike molecules composed of a series of nearly identical building blocks called nucleotides.
• Each nucleotide consists of a nitrogen-containing aromatic base attached to a pentose (five-carbon) sugar, which is in turn attached to a phosphate group

Explanation:

Biuret test - The biuret test is a chemical test that can be used to check for the presence of peptide bonds in a given analyte.

• Therefore, the biuret test can also be used to gauge the amount of protein present in the analyte.
• In this test, the presence of peptides results in the formation of pale purple-coloured coordination compounds of the copper (II) ion (when the solution is sufficiently alkaline). 
• The copper (II) present in the reaction binds itself to the nitrogen atoms that are present in the protein peptides. 

Therefore, the Biuret test is used to test the presence of Protein.