Chemistry Questions and Answers

Answer: D

The correct name is 3-Methylhexane.

The IUPAC rule is to take the longest continuous carbon chain as the parent. Starting from the end of the ethyl branch, the chain runs CH₃-CH₂-CH-CH₂-CH₂-CH₃, six carbons, so the parent is hexane. The remaining CH₃ is a methyl branch, and numbering from the end nearer to it puts it on carbon 3.

'2-Ethylpentane' and '1-Ethylpentane' come from taking the five-carbon chain as drawn, which is not the longest chain. '4-Methylpentane' is both the wrong chain and numbered from the wrong end.

Answer: C

The equilibrium constant is 1.79.

For \(\text{PCl}_5 \rightleftharpoons \text{PCl}_3 + \text{Cl}_2\), products go in the numerator and the reactant in the denominator, each raised to its coefficient (here 1):

\[ \begin{aligned} K_c &= \frac{[\text{PCl}_3][\text{Cl}_2]}{[\text{PCl}_5]} \\ &= \frac{1.59 \times 1.59}{1.41} \\ &= \frac{2.5281}{1.41} \\ &\approx 1.79 \end{aligned} \]

1.80 comes from rounding up too early, and 2.00 is a rough estimate. 1.00 would need the product of the two product concentrations to equal \([\text{PCl}_5]\), which it does not.

Answer: B

It is a neutralization reaction.

When an acid reacts with a base, the hydrogen ions of the acid combine with the hydroxide ions of the base to form water, and the remaining ions form a salt, for example HCl + NaOH gives NaCl + H₂O. Each cancels the other's acidic or basic character, hence the name.

A displacement reaction is one where a more reactive element pushes a less reactive one out of its compound. A redox reaction involves transfer of electrons (oxidation and reduction). Dissociation is the splitting of a compound into ions or smaller parts, not the combining of an acid and a base.

Answer: C

The true statements are (i), (iii) and (iv).

When lactic acid bacteria turn milk into curd, they convert lactose into lactic acid, which makes the milk proteins (casein) coagulate. So:

• (i) the milk changes from a liquid to a semi-solid,
• (iii) it tastes sour instead of sweet, and
• (iv) the change is a chemical one that cannot be reversed: curd cannot be turned back into milk.

Statement (ii) is not true, because curd stays white like the milk it came from. That rules out options including (ii), and the option naming only (ii) and (iii).

Answer: A

The change that is not chemical is ice to water.

Melting changes only the physical state of water from solid to liquid. No new substance forms, and freezing reverses it, so it is a physical change.

The others form new substances and cannot simply be reversed: milk turns to curd as bacteria convert lactose into lactic acid, burning coal produces carbon dioxide and ash, and making ghee from butter involves heating that changes the milk solids, which brown and give ghee its flavour.

Answer: A

The correct matching is a-IV, b-III, c-I, d-II.

• Oxygen is an element, a single kind of atom (IV).
• Water is a compound of hydrogen and oxygen in a fixed ratio (III).
• Sulphur in carbon disulphide is a solution, a homogeneous mixture with one uniform phase (I).
• Sugar mixed with salt is a heterogeneous mixture, since the two solids stay as separate visible particles (II).

Option D reverses the two mixtures, treating the solid sugar and salt mix as uniform.

Answer: B

An aqueous solution of potassium chloride is a strong electrolyte.

KCl is an ionic salt of a strong acid (HCl) and a strong base (KOH). When it dissolves in water it dissociates almost completely into K⁺ and Cl^- ions, so the solution conducts electricity well. Complete dissociation is exactly what defines a strong electrolyte.

A weak electrolyte, such as acetic acid or ammonia solution, ionises only partly. A non-electrolyte, such as sugar or urea, gives no ions at all. There is no stage at which a KCl solution is weak and later becomes strong; it is fully dissociated from the start.

Answer: B

Defining an acid as a substance that can lose (donate) a proton is the Bronsted-Lowry concept.

In 1923 Bronsted and Lowry independently defined an acid as a proton (H⁺) donor and a base as a proton acceptor, which extends acid-base behaviour beyond water.

The Arrhenius concept defines an acid as a substance that gives H⁺ ions in water. The Lewis concept defines an acid as an electron-pair acceptor. The Usanovich concept is the broadest of all, treating any substance that gives up cations or accepts anions or electrons as an acid. None of these three is phrased as 'losing a proton'.

Answer: B

The wrong statement is All the ores are not minerals.

A mineral is a naturally occurring substance found in the earth's crust. An ore is a mineral from which a metal can be extracted conveniently and economically. So every ore is a mineral, but not every mineral is an ore; for example, bauxite is both a mineral and the ore of aluminium, while clay contains aluminium yet is not worked as an ore.

That makes 'All the minerals are not ores' and 'All the ores are minerals' correct, and 'From ores, metals can be extracted economically' is the definition of an ore itself. Only the claim that ores are not minerals contradicts the definitions.

Answer: B

They are called molecular hydrides (covalent hydrides).

The p-block elements are mostly non-metals with electronegativities close to that of hydrogen, so they share electrons with it and form discrete covalent molecules such as CH₄, NH₃, H₂O and HF.

Ionic (saline) hydrides are formed by the highly electropositive s-block metals, such as NaH and CaH₂. Metallic (interstitial) hydrides are formed by d- and f-block elements. 'Polynuclear hydrides' is not one of the standard classes.