12th Standard Chemistry March 2020: Online Practice Test

Explanation. Fluorine is the strongest oxidising agent; the borax bead test identifies coloured metal ions; aluminium is the most abundant metal in the earth's crust; sulphur (a chalcogen) occurs in volcanic ash.

Explanation. Wolframite is magnetic while tinstone (cassiterite) is non-magnetic, so they are separated by electromagnetic separation.

Explanation. Scandium (3d¹4s²) loses all three valence electrons to give the stable Sc³⁺ ion, so it shows only the +3 oxidation state.

Explanation. Therapeutic index is the ratio of the lethal dose to the therapeutic dose; it measures the medicinal value (safety) of a drug.

Explanation. HCOONa and KCN are salts of a weak acid and a strong base, so they hydrolyse to give basic solutions; anilinium chloride is a salt of a weak base and a strong acid, so its solution is acidic.

Explanation. A pairs with T and G pairs with C, so ATGCTTGA pairs with TACGAACT.

Explanation. Methyl groups are electron-releasing and increase the electron density on nitrogen, while –Br, –Cl and –NO₂ withdraw electrons; hence 2,4-dimethyl aniline is the most basic.

Explanation. Mn changes from +7 in \(MnO_4^-\) to +2, gaining 5 electrons per ion, so 5 F are required per mole.

Explanation. As an aromatic aldehyde without α-hydrogen, p-N,N-dimethylaminobenzaldehyde takes part in (crossed) benzoin condensation, and –CHO is indeed meta directing, but its directing influence in ring substitution does not explain benzoin condensation. The item is loosely framed (the electron-donating –NMe₂ group hinders self-condensation); (c) is the intended answer.

Explanation. Laptops and mobile phones use light, rechargeable lithium-ion batteries.

Explanation. Hyponitrous acid is HO–N=N–OH, i.e. \(H_2N_2O_2\); \(HNO_2\) is nitrous acid.

Explanation. The methoxide ion attacks the methyl halide in a single step from the back side, displacing the halide: an \(S_N2\) reaction.

Explanation. The packing efficiency of a bcc unit cell is 68%, so the vacant space is 100 − 68 = 32%.

Explanation. By definition, the time taken for the concentration of a reactant to fall to half its initial value is the half-life period.

Explanation. Sulphonation of phenol at low temperature (about 280 K) gives mainly o-phenol sulphonic acid; at 373 K the para isomer predominates.