All Free Chemistry MCQs with Answers

491. A chemical reaction has a theoretical yield of 25 g, but only 20 g of product was obtained. What is the percentage yield of the reaction?

• A. 20%
• B. 25%
• C. 45%
• D. 80%

Explanation: Percentage yield is actual divided by theoretical multiplied by 100, so 20 over 25 multiplied by 100 gives 80 per cent. Dividing the other way round would give an impossible 125 per cent, which is the check worth making on every such answer. The missing 5 g is lost to transfer, side reactions or an incomplete reaction.

492. Which of the following situations most clearly demonstrates a key characteristic of a redox reaction?

• A. Water boiling to steam
• B. Hydrogen gas reacting with chlorine to form hydrogen chloride gas
• C. Sodium chloride dissolving in water
• D. Ethanol evaporating at room temperature

Explanation: Hydrogen goes from zero to plus 1 and chlorine from zero to minus 1, so electrons are transferred and oxidation numbers change, which is the defining feature of a redox reaction. Boiling, dissolving and evaporating are physical changes in which no oxidation number moves at all. Checking oxidation numbers on both sides is the reliable test.

493. If 100 kJ of heat is absorbed by the system and 40 kJ of work is done on the system, what is the change of internal energy?

• A. -60 kJ
• B. +60 kJ
• C. -140 kJ
• D. +140 kJ

Explanation: By the first law, the change in internal energy is the heat added to the system plus the work done on it, so 100 plus 40 gives plus 140 kJ. Both inputs raise the internal energy, which is why they add rather than cancel. Had the system done 40 kJ of work on the surroundings instead, the answer would have been plus 60 kJ, which is the distractor offered.

494. Number of unpaired electrons present in the ground state of Fe3+ are (Atomic number of Fe = 26).

• A. Three
• B. Four
• C. Five
• D. Six

Explanation: Neutral iron is [Ar] 3d6 4s2, and forming Fe3+ removes both 4s electrons and one 3d electron, leaving 3d5. By Hund's rule those five electrons occupy the five d orbitals singly with parallel spins, so all five are unpaired, and that half filled arrangement is what makes Fe3+ so stable. Removing d electrons before the 4s pair is the standard error.

495. If % yield and actual yield is 80 and 20 g respectively, what will be theoretical yield?

• A. 20 g
• B. 25 g
• C. 30 g
• D. 40 g

Explanation: Rearranging the percentage yield formula, the theoretical yield is the actual yield multiplied by 100 and divided by the percentage, so 20 times 100 over 80 gives 25 g. The theoretical yield must always exceed the actual yield, so any answer of 20 g or less can be rejected immediately. This is the reverse of the earlier question in the same paper.

496. Which of the following best identifies the redox reaction:

• A. Transfer of proton
• B. Transfer of electron
• C. Absorption of light
• D. Exchange of ions

Explanation: Oxidation is loss of electrons and reduction is gain, so a redox reaction is defined by electron transfer and by the change in oxidation number that accompanies it. Proton transfer defines an acid base reaction in the Bronsted sense, which is the closest distractor. Simple exchange of ions, as in a precipitation, involves no change in oxidation number at all.

497. Methanol is produced by reduction of

• A. Formaldehyde
• B. Acetaldehyde
• C. Propanal
• D. Propanone

Explanation: Reducing an aldehyde adds hydrogen across the carbonyl group and gives a primary alcohol with the same number of carbon atoms, so the one carbon aldehyde formaldehyde gives the one carbon alcohol methanol. Acetaldehyde would give ethanol and propanal would give propan-1-ol. Propanone is a ketone and would give a secondary alcohol, propan-2-ol.

498. IUPAC name of (CH3)3CCH2Br is

• A. 1-bromopentane
• B. 1-bromo-2,2,2-trimethylethane
• C. 1-bromo-2,2-dimethylpropane
• D. 2-bromopentane

Explanation: The longest chain containing the bromine has three carbons, giving propane, with the bromine on carbon 1 and two methyl groups on carbon 2. Counting all five carbons as a straight chain gives the pentane answers, which ignores the branching. Numbering must start from the end nearer the substituent, which puts the bromine at position 1.

499. Which one shows anisotropic behavior?

• A. Wood
• B. Gemstone
• C. Coke
• D. Graphite

Explanation: This is the option marked correct in the official UHS answer key. Graphite is crystalline with a strongly layered structure, so properties such as electrical conductivity differ enormously along the layers compared with across them, which is the essence of anisotropy. Coke is amorphous and therefore isotropic. Note that gemstones are crystalline and anisotropic too, and wood is anisotropic in the everyday sense of grain, so this question rests on the crystalline definition.

500. In a unit cell of a crystal lattice the angle beta is between faces:

• A. a and b
• B. b and c
• C. c and a
• D. Not specified

Explanation: By convention the three interfacial angles of a unit cell are alpha between the b and c axes, beta between c and a, and gamma between a and b, each angle being named after the axis it does not lie against. Together with the three edge lengths, these six parameters define the cell completely. The Greek letter did not print cleanly in the original paper, appearing there as a stray character.