Chemistry Questions and Answers

Answer: C

The salt of a weak acid and a strong base is NaCN.

Sodium cyanide comes from hydrocyanic acid (HCN, a weak acid) and sodium hydroxide (a strong base). Its solution is alkaline, because the cyanide ion takes a proton from water.

\(\text{NH}_{4}\text{Cl}\) is the salt of a strong acid and a weak base, so its solution is acidic. \(\text{CH}_{3}\text{COONH}_{4}\) comes from a weak acid and a weak base. NaCl comes from a strong acid and a strong base and is neutral.

Answer: C

Triptane is 2,2,3-trimethylbutane, a highly branched isomer of heptane with a very high octane rating, option C.

2,2,4-Trimethylpentane is iso-octane, 2,4,6-trimethylbenzene is mesitylene, and 1,2,3-trimethylbutane is not a valid name.

Triptane is the common name for 2,2,3-trimethylbutane, an isomer of heptane with the formula C7H16 and an octane number above 100. It is a branched hydrocarbon, which explains its antiknock property. 2,2,4-Trimethylpentane is the isooctane of the 100 point on the octane scale.

Answer: D

The oxoacid of chlorine with more oxygen atoms is stronger, so HClO3 has the highest dissociation constant, option D.

Among HClO, HBrO and HIO, the acidity falls as the halogen gets larger, and all of them are far weaker than HClO3.

For oxoacids of the same element, acidity increases with the number of oxygen atoms and the oxidation state of the central atom, so chloric acid is much stronger than hypochlorous acid. Among the hypohalous acids, acidity falls from HClO to HBrO and HIO, because the halogen gets less electronegative.

Answer: C

The true statements are (ii) and (iii). Cycloalkanes burn with a higher octane number than the straight-chain alkane of similar size, and alkenes and aromatic hydrocarbons also rate higher than straight-chain alkanes, so option (ii) and (iii) is right.

Statement (i) is false: for straight-chain alkanes the octane number falls as the chain gets longer (n-heptane is the zero of the scale). Statement (iv) is false because branching raises the octane number, which is why iso-octane is the 100 reference. Options with (i) or (iv) are therefore wrong.

Answer: D

Pure gold is 24 carat, which is not offered, so the answer is None of these.

The carat scale divides gold into 24 parts. 24 carat means all 24 parts are gold, that is, pure gold.

The listed values are alloys: 22 carat is 22/24, about 91.6% gold, the usual standard for jewellery in India; 21 carat is 87.5% and 18 carat is 75% gold. Each contains other metals, such as copper or silver, added to make it harder.

Answer: D

The chemical change is the rusting of iron.

In rusting, iron reacts with oxygen and water to form a new substance, hydrated iron(III) oxide, with properties quite different from iron. The change cannot be undone by simple physical means, which is the mark of a chemical change.

The other three are physical changes. Melting ice and turning water into steam are changes of state of the same substance, water, and dissolving sugar in water makes a solution from which the sugar can be recovered by evaporation. No new substance forms in any of them.

Answer: D

The one that is not an element is Marble.

An element is a substance made of only one kind of atom. Oxygen, nitrogen and hydrogen are all elements, found as gases made of two-atom molecules.

Marble is a rock, a metamorphic form of limestone made mostly of calcium carbonate, a compound of calcium, carbon and oxygen. So it is not an element.

Answer: A

Pencils use graphite.

The 'lead' of a pencil is in fact graphite, a soft form of carbon, mixed with clay and baked. Its layers slide over each other easily and leave a dark mark on paper. More clay gives a harder pencil (H grades), more graphite a softer, darker one (B grades).

Silicon and phosphorus are not used, and charcoal is used for drawing sticks rather than for ordinary pencils.

Answer: C

The gas is helium.

At the high pressures of deep dives, nitrogen dissolves in the blood in large amounts. It causes nitrogen narcosis, a drunken confusion, and can form bubbles on surfacing (the bends). Divers therefore breathe mixtures in which helium replaces some or all of the nitrogen, such as heliox and trimix, because helium is much less soluble and not narcotic.

Argon, xenon and krypton are inert too, but they are heavier and more narcotic under pressure than nitrogen, so they are not used.

Answer: D

Anthracite contains about 90%-98% carbon.

Anthracite is the highest grade of coal. It is hard and shiny, burns with a smokeless flame and has the highest carbon content and heating value, generally about 90 per cent or more.

The lower percentages belong to lower grades. Peat and lignite (brown coal) have much less carbon, and bituminous coal, the commonest grade, lies between lignite and anthracite, so the 40% to 65% ranges describe lower-grade coals or peat.