RRB JE CMA — Solved Practice Question Bank

Q: A body of mass 2 kg moving at 5 m/s is brought to rest by friction in 2 s. Find the friction force. Sol: a = (0−5)/2 = −2.5 m/s²; F = ma = 2×2.5 = 5 N (friction acts opposite to motion). Q: Two resistors of 6Ω and 3Ω are connected in parallel. Find the equivalent resistance. Sol: 1/R = 1/6 + 1/3 = 1/6 + 2/6 = 3/6; R = 2Ω. Q: A convex lens has focal length 20 cm. An object is placed 30 cm from it. Find image distance. Sol: 1/f = 1/v − 1/u → 1/20 = 1/v − 1/(−30) → 1/v = 1/20 − 1/30 = (3−2)/60 = 1/60 → v = 60 cm (real image).

Q: Calculate the molarity of a solution containing 4 g NaOH in 500 mL solution. (Molar mass NaOH = 40 g/mol) Sol: moles = 4/40 = 0.1; Molarity = 0.1/0.5 L = 0.2 M. Q: Which gives a positive test with Fehling's solution — aldehydes or ketones? Sol: Aldehydes (except aromatic ones) reduce Fehling's solution to form a brick-red Cu2O precipitate; ketones do not, since they lack the easily-oxidized aldehyde H. Q: Balance: Fe2O3 + CO → Fe + CO2 Sol: Fe2O3 + 3CO → 2Fe + 3CO2 (balances 2 Fe, 3 O from Fe2O3 side matched by 3 CO2 on product side, plus the 3 CO supplies exactly 3 C and 3 O).

Q: Why is roasting used for sulphide ores but calcination for carbonate ores? Sol: Roasting needs excess air/O2 to convert metal sulphide to metal oxide (releasing SO2) — carbonates have no sulphur to remove this way. Calcination just needs heat (no air required) to drive off CO2/H2O from a carbonate/hydrated ore. Q: In the blast furnace, what is the role of limestone? Sol: Limestone (CaCO3) decomposes to CaO, which acts as a basic flux, combining with acidic gangue (SiO2) to form fusible slag (CaSiO3) that floats above molten iron and is removed separately. Q: Name the process used to obtain ultra-pure silicon for semiconductors. Sol: Zone refining — a molten zone is moved along a silicon rod; since impurities are more soluble in the molten phase, they are swept to one end, leaving the rest ultra-pure.