ECAT — Engineering College Admission Test (UET)
Take the ECAT Chemistry — Periodic Table, s- and p-Block Elements (Set 5): 30 multiple-choice questions (40 minutes) with instant scoring and a full…
What this test covers
The first 5 questions of this test, with the correct answer and explanation.
Q1. Using Slater's rules (other electrons in the same ns, np group count 0.35 each; electrons in the n − 1 shell 0.85 each; deeper electrons 1.00 each), the effective nuclear charge on a 3p electron of a chlorine atom (Z = 17) is
Answer: A. 6.10
Explanation: Cl: 1s² 2s²2p⁶ 3s²3p⁵. Screening: 6 other n = 3 electrons × 0.35 = 2.10; 8 electrons in n = 2 × 0.85 = 6.80; 2 electrons in n = 1 × 1.00 = 2.00. σ = 10.90, so Z* = 17 − 10.90 = 6.10.
Q2. In Period 3 (Na to Ar), which two elements have a first ionisation energy lower than that of the element immediately before them?
Answer: D. Al and S
Explanation: Al (3p¹, a p electron is easier to remove than the Mg 3s² pair) and S (3p⁴, the paired p electron is repelled) are the dips: Mg 738 > Al 578 and P 1012 > S 1000 kJ mol⁻¹.
Q3. Gallium (Z = 31) has a slightly smaller atomic radius (about 135 pm) than aluminium (about 143 pm). The best explanation is that
Answer: B. the ten 3d electrons in Ga shield the nuclear charge poorly
Explanation: Between Al and Ga the d-block elements intervene. Their 3d electrons shield poorly, so the valence electrons of Ga feel a larger effective nuclear charge and the atom does not grow.
Q4. In aqueous solution the ionic mobility of the alkali-metal ions increases in the order Li⁺ < Na⁺ < K⁺ < Rb⁺ < Cs⁺. This is mainly because
Answer: B. the smaller ions are more heavily hydrated, so their hydrated radii are larger
Explanation: Li⁺ has the highest charge density and attracts the most water molecules, so the hydrated ion is the largest and moves slowest under an electric field.
Q5. When potassium burns in excess oxygen the main product is KO₂. Which statement about it is correct?
Answer: A. It is paramagnetic because O₂⁻ has one unpaired electron
Explanation: KO₂ is K⁺ O₂⁻. O₂⁻ has 17 electrons: σ²σ*²σ²π⁴π*³, giving one unpaired electron (bond order 1.5), so the compound is paramagnetic and coloured.
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Q6. Which products are formed when sodium nitrate is heated strongly?
Q7. Which of these Group 2 carbonates decomposes most easily on heating?
Q8. Gypsum, CaSO₄·2H₂O, is heated at 120 °C to give plaster of Paris, CaSO₄·½H₂O. What mass of gypsum (M = 172) is needed to prepare 14.5 g of plaster of Paris (M = 145)? (Ca = 40, S = 32, O = 16)
Q9. Hard water contains 81 mg dm⁻³ of Ca(HCO₃)₂ (M = 162) as its only hardness. Expressed as mg dm⁻³ of CaCO₃ (M = 100), the hardness is
Q10. In the Solvay process 117 g of NaCl is converted to Na₂CO₃ with an overall yield of 80 %. What mass of Na₂CO₃ is obtained? (Na₂CO₃ = 106; NaCl = 58.5)
Q11. In the diborane molecule, B₂H₆, how many three-centre two-electron (B–H–B) bonds are there?
Q12. 5.4 g of aluminium (Al = 27) is allowed to react completely with excess aqueous NaOH, giving Na[Al(OH)₄] and H₂. What volume of H₂ is produced at STP (22.4 dm³ mol⁻¹)?
Q13. Why is PbO₂ a strong oxidising agent whereas SnO₂ is not?
Q14. In the manufacture of silicones, which hydrolysed chloride acts as a chain terminator?
Q15. In a sheet (two-dimensional) silicate each SiO₄ tetrahedron shares three of its oxygen atoms. The formula of the anion per repeating unit is
Q16. Which pair of gases is the main constituent of water gas, produced by passing steam over red-hot coke?
Q17. In the Ostwald process the overall change is NH₃ + 2O₂ → HNO₃ + H₂O. What mass of oxygen is needed to make 126 g of HNO₃ at 100 % efficiency? (H = 1, N = 14, O = 16)
Q18. Phosphorous acid, H₃PO₃, is dibasic. What mass of NaOH (40 g mol⁻¹) is needed to neutralise 0.100 mol of H₃PO₃ completely?
Q19. Moist sulphur dioxide bleaches coloured flowers, but the colour slowly returns on standing in air, whereas bleaching by chlorine is permanent. This is because
Q20. Hot concentrated NaOH reacts with chlorine: 3Cl₂ + 6NaOH → 5NaCl + NaClO₃ + 3H₂O. How many moles of NaClO₃ are formed from 0.60 mol of Cl₂?
Q21. Which is the correct order of acid strength of the hypohalous acids?
Q22. The interhalogen compound IF₇ has the shape and central-atom hybridisation
Q23. Bartlett prepared the first noble-gas compound, Xe⁺[PtF₆]⁻, after noticing that
Q24. What volume of hydrogen at STP (22.4 dm³ mol⁻¹) is released when 4.2 g of calcium hydride (CaH₂, M = 42) reacts completely with water?
Q25. Which of the following is a neutral oxide?
Q26. Aluminium carbide reacts with water: Al₄C₃ + 12H₂O → 4Al(OH)₃ + 3CH₄. How many moles of methane form from 0.50 mol of Al₄C₃? (Be₂C reacts similarly, one example of the Be–Al diagonal relationship.)
Q27. A newly made element has atomic number 115. Given the pattern of filling (…7s² 5f¹⁴ 6d¹⁰ 7p³), it should be placed in
Q28. The first electron affinities (kJ mol⁻¹ released) are C 122, N about −7 (essentially zero), O 141 and F 328. Why is the value for nitrogen so much lower than those of its neighbours?
Q29. In the dimer Al₂Cl₆, how many chlorine atoms are bridging, and how do the bridges form?
Q30. 16.4 g of Ca(NO₃)₂ (M = 164) is decomposed completely: 2Ca(NO₃)₂ → 2CaO + 4NO₂ + O₂. What volume of gas is produced at STP? (22.4 dm³ mol⁻¹; treat NO₂ as an ideal gas and ignore dimerisation)