CBSE Class 10 Chemistry Carbon and its Compounds MCQs Set 19

Answer: (c) Ethanal

Explanation:
1. The compound has two carbons including the aldehyde carbon, so the stem is eth-.
2. The suffix -al gives ethanal. Acetaldehyde is the common name.

Teacher's Note:
1. IUPAC names use the parent chain and a suffix.
2. Methanal is \( \ce{HCHO} \).

Answer: (a) 2,2,3-Trimethylpentane

Explanation:
1. The longest chain has 5 carbons, so the parent is pentane.
2. Number from the end that gives the lowest locants: methyl groups at 2, 2 and 3.
3. The name is 2,2,3-trimethylpentane.

Teacher's Note:
1. Always choose the numbering that gives the lowest set of locants.
2. 3,4,4 is higher than 2,2,3.

Answer: (b) Propene

Explanation:
1. The first alkene is ethene, \( \ce{C2H4} \).
2. The second is propene, \( \ce{C3H6} \).

Teacher's Note:
1. There is no alkene with one carbon.
2. Ethyne is an alkyne.

Answer: (a) 3-Methylpentanoyl chloride

Explanation:
1. The longest chain including the -COCl carbon has 5 carbons, so the parent is pentane.
2. The suffix for -COCl is -oyl chloride and the methyl group is at carbon 3.
3. The name is 3-methylpentanoyl chloride.

Teacher's Note:
1. The acid chloride carbon is always carbon 1.
2. Count from the COCl carbon.

Answer: (b) substitution reaction

Explanation:
1. One hydrogen atom of methane is replaced by a chlorine atom.
2. Replacement of one atom by another is a substitution reaction.

Teacher's Note:
1. Alkanes mainly undergo substitution, alkenes undergo addition.
2. Sunlight is needed to start the reaction.

Answer: (b) \( \ce{CCl4} \)

Explanation:
1. Substitution continues step by step: \( \ce{CH3Cl} \), \( \ce{CH2Cl2} \), \( \ce{CHCl3} \) and finally \( \ce{CCl4} \).
2. With excess chlorine all four hydrogens are replaced.

Teacher's Note:
1. The reaction goes through four steps.
2. CCl4 is carbon tetrachloride.

Answer: (b) \( \ce{(CH3)2C=CH-CH3} \)

Explanation:
1. But-2-ene has the double bond between carbons 2 and 3 of a four carbon chain.
2. A methyl group on carbon 2 gives \( \ce{(CH3)2C=CH-CH3} \).
3. The other structures are 2-methylbutane, 2-methylbut-1-ene and 3-methylbut-1-ene.

Teacher's Note:
1. Draw the main chain, place the double bond, then add the branch.
2. The double bond must be between carbons 2 and 3.

Answer: (c) \( \ce{CH3CH2CH3} \)

Explanation:
1. Chlorination of methane gives \( \ce{CH3Cl} \), \( \ce{CH2Cl2} \), \( \ce{CHCl3} \) and \( \ce{CCl4} \).
2. Propane has three carbons and cannot form from a one carbon compound in this reaction.

Teacher's Note:
1. Substitution does not change the carbon skeleton.
2. Only chloromethanes and HCl form.

Answer: (d) \( \ce{CO2} \) and \( \ce{H2O} \)

Explanation:
1. In plenty of air, complete combustion takes place.
2. The carbon forms \( \ce{CO2} \) and the hydrogen forms \( \ce{H2O} \).

Teacher's Note:
1. Limited air gives carbon monoxide or soot.
2. Complete combustion needs excess oxygen.

Answer: (c) 3

Explanation:
1. \( \ce{C2H5OH + 3O2 -> 2CO2 + 3H2O} \).
2. Three molecules of oxygen are needed.

Teacher's Note:
1. Balance carbon first, then hydrogen, then oxygen.
2. Ethanol already has one oxygen atom.

Answer: (b) non-sooty flame

Explanation:
1. Saturated hydrocarbons have a low percentage of carbon and burn completely in air.
2. So the flame is clean, blue and non-sooty.

Teacher's Note:
1. Unsaturated hydrocarbons burn with a yellow sooty flame.
2. Complete combustion gives only \( \ce{CO2} \) and \( \ce{H2O} \).

Answer: (b) Ethanoic acid

Explanation:
1. Turning blue litmus red and reacting with bicarbonate shows a carboxylic acid.
2. The formula \( \ce{C2H4O2} \) has two carbons, so it is ethanoic acid, \( \ce{CH3COOH} \).

Teacher's Note:
1. Carboxylic acids release \( \ce{CO2} \) from bicarbonates.
2. Methanoic acid is \( \ce{CH2O2} \).

Answer: (a) pure acetic acid containing almost no water

Explanation:
1. Pure ethanoic acid freezes to an ice-like solid at about 290 K, which is why it is called glacial.
2. A 5 to 8 percent solution is vinegar.

Teacher's Note:
1. Glacial means ice-like.
2. Vinegar is the dilute solution.

Answer: (a) hot concentrated \( \ce{H2SO4} \)

Explanation:
1. Hot concentrated sulphuric acid removes water from ethanol (dehydration) at about 443 K.
2. \( \ce{C2H5OH -> C2H4 + H2O} \).

Teacher's Note:
1. Concentrated sulphuric acid acts as a dehydrating agent.
2. Alkaline KMnO4 oxidises ethanol to ethanoic acid.

Answer: (a) a very weak acid

Explanation:
1. Sodium replaces the hydrogen of the O-H group and hydrogen gas is evolved.
2. Release of H from -OH like this is typical of a very weak acid.

Teacher's Note:
1. Ethanol is neutral to litmus although it reacts with sodium.
2. Only active metals like sodium react in this way.

Answer: (b) \( \ce{CH3-CO-OCH2CH3} \)

Explanation:
1. The name ethanoate comes from ethanoic acid, so the acid part is \( \ce{CH3CO-} \).
2. Ethyl is the alcohol part, \( \ce{-OCH2CH3} \).

Teacher's Note:
1. Name an ester as alkyl alkanoate.
2. The first word is from the alcohol and the second from the acid.

Answer: (c) \( \ce{CH4} \)

Explanation:
1. Heating sodium ethanoate with soda lime removes the carboxyl group as sodium carbonate.
2. \( \ce{CH3COONa + NaOH -> CH4 + Na2CO3} \).
3. The gas is methane.

Teacher's Note:
1. This is decarboxylation, which gives an alkane with one carbon less.
2. The product has one carbon less than the acid.

Answer: (a) glycerol

Explanation:
1. Saponification of fats and oils with NaOH gives soap and glycerol.
2. Glycerol is therefore the by-product.

Teacher's Note:
1. Fat plus alkali gives soap plus glycerol.
2. Glycerol is used in cosmetics and medicines.

Answer: (b) \( \ce{C2H6} \)

Explanation:
1. Hydrogenation adds \( \ce{H2} \) across the double bond.
2. \( \ce{CH2=CH2 + H2 -> CH3-CH3} \) with a nickel catalyst.

Teacher's Note:
1. Hydrogenation turns an alkene into an alkane.
2. Vegetable oils are hydrogenated into ghee by the same idea.

Answer: (d) All of these

Explanation:
1. Hardness is caused by dissolved calcium and magnesium salts such as chlorides, sulphates and bicarbonates.
2. All three salts make water hard.

Teacher's Note:
1. Hard water does not give lather easily with soap.
2. Boiling removes only temporary hardness due to bicarbonates.