Chemistry

191. Which one of the following is the chemical formula of Plaster of Paris

Which one of the following is the chemical formula of Plaster of Paris ?

[amp_mcq option1=”$\text{CaSO}_4 \cdot \frac{1}{2} \text{ H}_2\text{O}$” option2=”$\text{CaSO}_4 \cdot 2 \text{ H}_2\text{O}$” option3=”$\text{CaSO}_4 \cdot 5 \text{ H}_2\text{O}$” option4=”$\text{CaSO}_4 \cdot 4 \text{ H}_2\text{O}$” correct=”option1″]

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The correct option is A. The chemical formula of Plaster of Paris is Calcium Sulfate hemihydrate, which is $\text{CaSO}_4 \cdot \frac{1}{2} \text{ H}_2\text{O}$.
Plaster of Paris is produced by heating Gypsum ($\text{CaSO}_4 \cdot 2 \text{ H}_2\text{O}$) to about 150°C (302°F). This heating process removes three-quarters of the water of crystallization, leaving the hemihydrate form. When mixed with water, Plaster of Paris rehydrates and sets into a hard mass, which is chemically Gypsum.
Gypsum ($\text{CaSO}_4 \cdot 2 \text{ H}_2\text{O}$) is Calcium Sulfate dihydrate. It is a naturally occurring mineral. The setting of Plaster of Paris is an exothermic reaction: $\text{CaSO}_4 \cdot \frac{1}{2} \text{ H}_2\text{O} + 1\frac{1}{2} \text{ H}_2\text{O} \rightarrow \text{CaSO}_4 \cdot 2 \text{ H}_2\text{O}$.

192. What is the total number of covalent bonds in methanol ?

What is the total number of covalent bonds in methanol ?

[amp_mcq option1=”3″ option2=”4″ option3=”5″ option4=”6″ correct=”option3″]

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The correct option is C. Methanol has the chemical formula $\text{CH}_3\text{OH}$. To find the number of covalent bonds, we need to consider its molecular structure.
The structure of methanol involves a central carbon atom bonded to three hydrogen atoms, one oxygen atom, and the oxygen atom is bonded to one hydrogen atom.
Drawing the Lewis structure or structural formula:
Carbon (C) forms single bonds with 3 Hydrogen atoms (H) and 1 Oxygen atom (O).
Oxygen (O) forms a single bond with the Carbon atom (C) and a single bond with 1 Hydrogen atom (H).
The bonds are: 3 C-H single bonds, 1 C-O single bond, and 1 O-H single bond.
Total number of covalent bonds = 3 (C-H) + 1 (C-O) + 1 (O-H) = 5.
Each line in the structural formula represents a single covalent bond, which is a pair of shared electrons. Carbon typically forms 4 bonds, Oxygen typically forms 2 bonds and has 2 lone pairs, and Hydrogen forms 1 bond. This structure satisfies the valency rules for all atoms.

193. Which one of the following ions is not iso-electronic with F⁻ ?

Which one of the following ions is not iso-electronic with F⁻ ?

[amp_mcq option1=”O²⁻” option2=”Na⁺” option3=”Ne” option4=”N⁻” correct=”option4″]

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The correct option is D. Iso-electronic species are atoms or ions that have the same number of electrons. We need to find which species among the options does not have the same number of electrons as F⁻.
A neutral Fluorine atom (F) has 9 electrons (atomic number = 9). The F⁻ ion has gained one electron, so it has $9 + 1 = 10$ electrons.
Let’s find the number of electrons for each option:
A) O²⁻: Neutral Oxygen (O) has 8 electrons (atomic number = 8). O²⁻ has gained two electrons, so $8 + 2 = 10$ electrons.
B) Na⁺: Neutral Sodium (Na) has 11 electrons (atomic number = 11). Na⁺ has lost one electron, so $11 – 1 = 10$ electrons.
C) Ne: Neutral Neon (Ne) has 10 electrons (atomic number = 10).
D) N⁻: Neutral Nitrogen (N) has 7 electrons (atomic number = 7). N⁻ has gained one electron, so $7 + 1 = 8$ electrons.
Thus, N⁻ has 8 electrons, while F⁻ has 10 electrons. N⁻ is not isoelectronic with F⁻.
Isoelectronic species often have similar chemical and physical properties due to having the same electron configuration. The species O²⁻, F⁻, Ne, Na⁺, Mg²⁺, Al³⁺ are all isoelectronic with 10 electrons, having the electron configuration of Neon.

194. Cinnabar is an ore of which one of the following ?

Cinnabar is an ore of which one of the following ?

[amp_mcq option1=”Copper” option2=”Zinc” option3=”Mercury” option4=”Manganese” correct=”option3″]

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Cinnabar is a mineral that is the chief ore of mercury. Its chemical composition is mercury(II) sulfide (HgS). It is known for its distinctive bright red color and is historically important as a source of mercury metal.
Cinnabar is the main ore from which mercury (Hg) is extracted. It is chemically represented as HgS.
Copper ores include chalcopyrite ($\text{CuFeS}_2$) and malachite ($\text{Cu}_2\text{CO}_3(\text{OH})_2$). Zinc ores include sphalerite (ZnS) and calamine (a historical term for zinc ores, including smithsonite ($\text{ZnCO}_3$) and hemimorphite ($\text{Zn}_4\text{Si}_2\text{O}_7(\text{OH})_2 \cdot \text{H}_2\text{O}$)). Manganese ores include pyrolusite ($\text{MnO}_2$).

195. To protect steel and iron from rusting, a thin layer of which one of t

To protect steel and iron from rusting, a thin layer of which one of the following metals is applied ?

[amp_mcq option1=”Magnesium” option2=”Zinc” option3=”Aluminium” option4=”Lead” correct=”option2″]

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Rusting is the process of oxidation of iron, forming hydrated iron(III) oxide. To protect iron and steel from rusting, a protective layer of another metal is applied. Zinc is commonly used for this purpose through a process called galvanization. Zinc is more reactive than iron, and it preferentially corrodes, acting as a sacrificial layer to protect the underlying iron or steel even if the coating is scratched.
Galvanization, the process of coating iron or steel with a thin layer of zinc, is a widely used method to prevent rusting. Zinc acts as a barrier and also provides sacrificial protection (cathodic protection) because it is more electrochemically active than iron.
Other methods to prevent rusting include painting, oiling, greasing, chrome plating, tin plating, and alloying (like making stainless steel). Aluminium is also more reactive than iron and forms a protective oxide layer, but galvanization with zinc is the most common method among the options listed for general anti-rust protection of steel and iron.

196. Which one of the following elements’ isotopes is used in the treatment

Which one of the following elements’ isotopes is used in the treatment of cancer ?

[amp_mcq option1=”Iodine” option2=”Sodium” option3=”Cobalt” option4=”Uranium” correct=”option3″]

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Radioactive isotopes are used in medicine for diagnosis and treatment. In the treatment of cancer, a process called radiotherapy is often used, which involves using radiation to kill cancer cells. Cobalt-60 is a radioactive isotope that emits high-energy gamma rays. These gamma rays can penetrate tissue and are used in external beam radiotherapy machines (often called “Cobalt therapy”) to target and destroy cancerous tumors.
Cobalt-60 ($^{60}$Co) is a common radioactive isotope used in cancer treatment (radiotherapy) due to its emission of gamma rays. While Iodine-131 is used to treat thyroid cancer, Cobalt-60 is used for a broader range of cancers in external radiotherapy.
Other radioisotopes used in medicine include Iodine-131 (thyroid conditions), Technetium-99m (diagnostic imaging), and Iridium-192 (brachytherapy). Uranium isotopes are mainly used as nuclear fuel and in nuclear weapons, not typically in medical treatment.

197. Chlorine occurs in nature in two isotopic forms of masses 35 u and 37

Chlorine occurs in nature in two isotopic forms of masses 35 u and 37 u in the ratio of 3 : 1 respectively. What is the average atomic mass of the Chlorine atom ?

[amp_mcq option1=”36·1 u” option2=”35·5 u” option3=”36·5 u” option4=”35·1 u” correct=”option2″]

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Chlorine exists as two isotopes: $^{35}$Cl with a mass of 35 u and $^{37}$Cl with a mass of 37 u. They are present in nature in a ratio of 3:1. This means that out of every 4 chlorine atoms, approximately 3 have a mass of 35 u and 1 has a mass of 37 u. The average atomic mass is calculated as a weighted average of the masses of the isotopes based on their relative abundance. The relative abundance of $^{35}$Cl is 3/4 or 0.75, and the relative abundance of $^{37}$Cl is 1/4 or 0.25.
Average atomic mass = ($\text{Mass of isotope 1} \times \text{Fractional abundance of isotope 1}$) + ($\text{Mass of isotope 2} \times \text{Fractional abundance of isotope 2}$)
Average atomic mass = $(35 \text{ u} \times 3/4) + (37 \text{ u} \times 1/4)$
Average atomic mass = $(105/4) \text{ u} + (37/4) \text{ u}$
Average atomic mass = $(105 + 37)/4 \text{ u}$
Average atomic mass = $142/4 \text{ u}$
Average atomic mass = $35.5 \text{ u}$.
Atomic mass unit (u) is a standard unit used for indicating atomic and molecular masses. Average atomic mass is the value listed for an element on the periodic table. It reflects the natural isotopic composition of the element.

198. What is the maximum number of electrons in the M-Shell ?

What is the maximum number of electrons in the M-Shell ?

[amp_mcq option1=”6″ option2=”8″ option3=”18″ option4=”32″ correct=”option3″]

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Electron shells are denoted by letters K, L, M, N, … or by principal quantum numbers n = 1, 2, 3, 4, … The K-shell corresponds to n=1, the L-shell to n=2, the M-shell to n=3, and so on. The maximum number of electrons that can occupy a shell with principal quantum number ‘n’ is given by the formula $2n^2$. For the M-shell, n=3, so the maximum number of electrons is $2 \times (3)^2 = 2 \times 9 = 18$.
The formula for the maximum number of electrons in a shell with principal quantum number ‘n’ is $2n^2$.
K-shell (n=1): $2 \times 1^2 = 2$ electrons.
L-shell (n=2): $2 \times 2^2 = 8$ electrons.
M-shell (n=3): $2 \times 3^2 = 18$ electrons.
N-shell (n=4): $2 \times 4^2 = 32$ electrons.
While the formula $2n^2$ gives the maximum capacity of a shell, the actual filling of electrons follows the Aufbau principle, Hund’s rule, and the Pauli exclusion principle, which dictates the filling order based on subshells (s, p, d, f) and energy levels. The outermost shell cannot accommodate more than 8 electrons (octet rule, with some exceptions), and the second last shell cannot accommodate more than 18 electrons.

199. Which one of the following methods can be used to separate anthracene

Which one of the following methods can be used to separate anthracene from a mixture of salt and anthracene ?

[amp_mcq option1=”Distillation” option2=”Sublimation” option3=”Evaporation” option4=”Chromatography” correct=”option2″]

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Anthracene is a polycyclic aromatic hydrocarbon known for its property of sublimation, which is the process of transitioning directly from the solid to the gas phase upon heating, without passing through a liquid phase. Salt (e.g., sodium chloride) is an ionic compound with a very high melting point and does not sublime under normal conditions. Therefore, sublimation is an effective method to separate anthracene from a mixture containing salt. By heating the mixture, anthracene will turn into a gas and can be collected by cooling the vapour, leaving the non-sublimable salt behind.
– Anthracene sublimes easily upon heating.
– Salt does not sublime under typical conditions.
– Sublimation is used to separate a sublimable solid from non-sublimable solids.
Other methods like distillation and evaporation are generally used for separating liquids or separating dissolved solids from solvents. Chromatography is a technique for separating components of a mixture based on different affinities for a stationary and mobile phase, which could be used but is not as direct or efficient as sublimation for this specific type of mixture.

200. Which one of the following pairs of elements is liquid at room tempera

Which one of the following pairs of elements is liquid at room temperature and at normal pressure ?

[amp_mcq option1=”Gallium and Bromine” option2=”Mercury and Bromine” option3=”Gallium and Mercury” option4=”Gallium and Caesium” correct=”option2″]

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At standard room temperature (approximately 25°C) and normal pressure, only two elements are liquids: Mercury (Hg), a metal, and Bromine (Br₂), a non-metal.
– Mercury is the only metallic element that is liquid at room temperature.
– Bromine is the only non-metallic element that is liquid at room temperature.
– Gallium (melting point 29.8 °C) and Caesium (melting point 28.5 °C) melt just above typical room temperature but are usually considered solid at 25°C.
While Gallium and Caesium have very low melting points and can melt in slightly warmer conditions or even in the hand, Mercury and Bromine are consistently liquid under standard room temperature conditions.
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