835 practice questions for CBSE Class 12 Chemistry, organised by NCERT chapter (10 chapters). Every question includes a written explanation. Pick a chapter below, or try the samples first.
Butane, propanal, propan-1-ol and ethanoic acid all have molar masses between $58$ and $60\ \text{g mol}^{-1}$. Which of them has the highest boiling point?
AButane
BEthanoic acid
CPropanal
DPropan-1-ol
Show answer and explanation
Correct answer: B - Ethanoic acid
Ethanoic acid molecules pair up through two hydrogen bonds into a dimer, so the strongest intermolecular attraction of the four must be broken before it boils. Propan-1-ol hydrogen bonds as well, but each molecule offers only one $\text{O-H}$ hydrogen, so its attraction is weaker. Propanal is polar yet has no $\text{O-H}$ and manages only dipole-dipole attraction, and butane is held together by weak dispersion forces alone.
Q2
easy
Benzaldehyde, $\text{C}_6\text{H}_5\text{CHO}$, is to be made by the Rosenmund reduction. Which compound should be hydrogenated over $\text{Pd/BaSO}_4$?
Correct answer: A - Benzoyl chloride, $\text{C}_6\text{H}_5\text{COCl}$
The Rosenmund reduction turns an acyl chloride $\text{RCOCl}$ into $\text{RCHO}$, so the starting material must carry chlorine on a carbonyl carbon, as benzoyl chloride does. In benzyl chloride the chlorine sits on a $\text{CH}_2$ group, so hydrogenation cannot build a $\text{CHO}$ group. Chlorobenzene holds its chlorine on the ring and is untouched, and a carboxylic acid is not reduced by hydrogen over poisoned palladium.
Q3
easy
Acetone is the common name of the simplest ketone, $\text{CH}_3\text{COCH}_3$. What is its IUPAC name?
APropanal
BButanone
CPropanone
DEthanal
Show answer and explanation
Correct answer: C - Propanone
Acetone is a three-carbon chain with the carbonyl on carbon 2, so its IUPAC name is propanone. Propanal has the same three carbons but the oxygen at the end of the chain, which makes it an aldehyde rather than a ketone. Ethanal is the IUPAC name of acetaldehyde, a different common name. Butanone has four carbons and is the IUPAC name of methyl ethyl ketone.
Q4
hard
In acidic medium, $0.02\ \text{mol}$ of $\text{K}_2\text{Cr}_2\text{O}_7$ is used to oxidise $\text{Fe}^{2+}$ to $\text{Fe}^{3+}$. How many moles of $\text{Fe}^{2+}$ react completely?
A$0.10\ \text{mol}$
B$0.12\ \text{mol}$
C$0.02\ \text{mol}$
D$0.06\ \text{mol}$
Show answer and explanation
Correct answer: B - $0.12\ \text{mol}$
The half-reaction $\text{Cr}_2\text{O}_7^{2-} + 14\text{H}^+ + 6e^- \rightarrow 2\text{Cr}^{3+} + 7\text{H}_2\text{O}$ shows that each dichromate ion accepts six electrons, and each $\text{Fe}^{2+}$ supplies one. So the iron(II) needed is $6 \times 0.02 = 0.12$ mol. Using five electrons, the permanganate value, would give $0.10$ mol; three electrons would give $0.06$ mol; a one-to-one ratio would give $0.02$ mol.
Q5
medium
A current of $1.5\ \text{A}$ is passed through an aqueous copper sulphate solution for $10$ minutes. Taking the molar mass of copper as $63.5\ \text{g mol}^{-1}$ and $F = 96500\ \text{C mol}^{-1}$, what mass of copper is deposited at the cathode?
A$0.0049\ \text{g}$
B$0.148\ \text{g}$
C$0.296\ \text{g}$
D$0.592\ \text{g}$
Show answer and explanation
Correct answer: C - $0.296\ \text{g}$
The charge passed is $Q = It = 1.5 \times 600 = 900\ \text{C}$, and depositing copper needs two electrons, so $m = \frac{63.5}{2 \times 96500} \times 900 = 0.296\ \text{g}$. Treating copper as needing one electron doubles this to $0.592\ \text{g}$, and using four electrons halves it to $0.148\ \text{g}$. Leaving the time in minutes gives only $15\ \text{C}$ and an answer of $0.0049\ \text{g}$.
Q6
easy
Which of the following species cannot act as a ligand towards a metal ion?
A$\text{CH}_4$
B$\text{NH}_3$
C$\text{H}_2\text{O}$
D$\text{CN}^-$
Show answer and explanation
Correct answer: A - $\text{CH}_4$
A ligand must carry at least one lone pair to donate into an empty metal orbital. In methane all four valence pairs of carbon are used up in $\text{C}-\text{H}$ bonds, so nothing is left to donate. Water has lone pairs on oxygen, ammonia one on nitrogen and the cyanide ion one on carbon, so all three bind metals readily.
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