All Free Chemistry MCQs with Answers

471. Oxidation numbers of X, Y, Z are +6, -2 and -1 respectively. Possible molecular formula when atoms combine:

• A. X2YZ
• B. XY2Z2
• C. XYZ2
• D. XY2Z

Explanation: A neutral compound must have oxidation numbers summing to zero, and XY2Z2 gives plus 6, then two at minus 2 and two at minus 1, that is 6 minus 4 minus 2, which is exactly zero. Testing each option the same way disposes of the others: XYZ2 gives 6 minus 2 minus 2, which is plus 2 rather than zero. Summing to zero is the only test needed for a neutral formula.

472. Which set of quantum numbers is not allowed for an electron?

• A. n=2, l=1, m=0, s=+1/2
• B. n=3, l=0, m=0, s=-1/2
• C. n=2, l=2, m=1, s=+1/2
• D. n=1, l=0, m=0, s=+1/2

Explanation: The azimuthal quantum number can only run from 0 up to n minus 1, so with n equal to 2 the value of l cannot reach 2, and the set describes a non existent 2d orbital. The other three sets satisfy every restriction, including that m lies between minus l and plus l. Checking l against n first, then m against l, resolves questions of this type quickly.

473. 2A + B gives 3C + D. Which of the following does not express the reaction rate?

• A. -d[B]/dt
• B. d[D]/dt
• C. -1/2 d[A]/dt
• D. -1/3 d[C]/dt

Explanation: Rate is expressed as the change in concentration divided by the coefficient, with a minus sign for reactants because they are being consumed and a plus sign for products because they are being formed. C is a product, so its term must be positive, and writing it with a minus sign makes the expression wrong. The other three all give the same positive rate.

474. The order of ease of reduction of H+1, Cu+2 and Ag+1:

• A. H+1 > Cu+2 > Ag+
• B. H+1 > Ag+ > Cu+2
• C. Ag+ > Cu+2 > H+1
• D. H+1 > Cu+2 > Ag+

Explanation: The more positive the standard electrode potential, the more readily the ion accepts electrons, and the values are plus 0.80 V for silver, plus 0.34 V for copper and zero for hydrogen by definition. Silver is therefore reduced most easily and hydrogen least, which is also why copper and silver do not displace hydrogen from dilute acids. Note that options (a) and (d) were printed identically in the original paper.

475. Acetaldehyde undergoes oxidation to produce acetic acid. In this reaction the oxidizing agent used is:

• A. HI
• B. LiAlH4
• C. K2Cr2O7
• D. NaBH4

Explanation: Acidified potassium dichromate is the standard oxidising agent for converting an aldehyde to a carboxylic acid, turning from orange to green as chromium is reduced from plus 6 to plus 3. Lithium aluminium hydride and sodium borohydride are reducing agents and would drive the reaction the other way, back towards the alcohol. Hydrogen iodide is a reducing agent as well.

476. Which one of the following has higher vapour pressure?

• A. Acetone
• B. Acetaldehyde
• C. Isopentane
• D. Benzene

Explanation: This is the option marked correct in the official UHS answer key. Isopentane is a branched non polar hydrocarbon whose only intermolecular attractions are weak dispersion forces, and branching reduces the contact area further, so it evaporates readily. Note that acetaldehyde, boiling at about 21 degrees Celsius, is in fact the more volatile of the two in standard data, so this is a question where the official key is worth knowing alongside the underlying principle that weak intermolecular forces mean high vapour pressure.

477. Terminal alkynes show acidic character due to:

• A. Terminal carbon atoms are sp hybridized
• B. Terminal carbon atoms are sp2 hybridized
• C. Terminal carbon atoms are sp3 hybridized
• D. Terminal carbon atoms show hydrogen bonding

Explanation: An sp hybrid orbital has 50 per cent s character, so it holds the bonding electrons closer to the carbon nucleus and the attached hydrogen is released more easily as a proton. This is why ethyne gives a precipitate with ammoniacal silver nitrate while ethene and ethane do not. Acidity therefore rises with s character in the order sp3 to sp2 to sp.

478. Fischer esterification is which type of reaction:

• A. Condensation
• B. Substitution
• C. Elimination
• D. Dehydrogenation

Explanation: A carboxylic acid and an alcohol join with the loss of a small molecule, water, which is the definition of a condensation, and concentrated sulphuric acid catalyses it while also shifting the equilibrium by removing that water. The reaction is reversible, so the yield is never complete. Nucleophilic acyl substitution describes the mechanism, but the reaction type the question asks for is condensation.

479. Shape of orbital is determined by quantum number:

• A. n
• B. l
• C. m
• D. s

Explanation: The azimuthal quantum number l fixes the subshell and therefore the shape, with l equal to 0 giving a spherical s orbital and l equal to 1 giving a dumb-bell shaped p orbital. The principal quantum number n sets the main energy level and size, m sets the orientation in space and s the direction of electron spin. Knowing which quantum number controls which property answers a whole family of questions.

480. The theories which explain the formation of sigma and pi bond are EXCEPT:

• A. MOT
• B. VBT
• C. VSEPR
• D. CFT

Explanation: Crystal field theory deals with how the d orbitals of a transition metal ion are split by the surrounding ligands, so it explains colour and magnetic behaviour in complexes rather than the formation of sigma and pi bonds. Valence bond theory and molecular orbital theory both describe those bonds directly through orbital overlap. VSEPR predicts shape from electron pair repulsion and so also concerns bonding.