ECAT — Engineering College Admission Test (UET)
Take the ECAT Chemistry — Stoichiometry and Atomic Structure (Set 3): 30 multiple-choice questions (40 minutes) with instant scoring and a full answer…
What this test covers
The first 5 questions of this test, with the correct answer and explanation.
Q1. How many oxygen atoms are present in 4.9 g of H₂SO₄ (molar mass 98 g/mol)?
Answer: C. 1.204 × 10²³
Explanation: 4.9/98 = 0.05 mol; each molecule has 4 O atoms so 0.20 mol × 6.02 × 10²³ = 1.204 × 10²³.
Q2. What mass of calcium is present in 250 g of pure CaCO₃? (Ca = 40, C = 12, O = 16)
Answer: C. 100 g
Explanation: CaCO₃ is 40/100 calcium by mass: 250 × 0.40 = 100.0 g.
Q3. 3.0 g of ethane (C₂H₆) is burned completely in excess oxygen: 2C₂H₆ + 7O₂ → 4CO₂ + 6H₂O. What volume of CO₂ is produced at STP?
Answer: D. 4.48 L
Explanation: 3.0/30 = 0.1 mol C₂H₆ gives 0.2 mol CO₂ (ratio 2:4); 0.2 × 22.4 = 4.48 L.
Q4. On heating 24.95 g of hydrated copper sulphate CuSO₄·xH₂O, 15.95 g of anhydrous salt remains (molar mass 159.5 g/mol). What is x?
Answer: C. 5
Explanation: Water lost = 9.00 g = 0.5 mol; salt = 15.95/159.5 = 0.1 mol; x = 0.5/0.1 = 5.
Q5. An oxide of iron contains 70% iron and 30% oxygen by mass. Its empirical formula is (Fe = 56, O = 16):
Answer: D. Fe₂O₃
Explanation: Fe: 70/56 = 1.25; O: 30/16 = 1.875; ratio 1 : 1.5 = 2 : 3, so Fe₂O₃.
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Q6. 12 g of H₂ and 64 g of O₂ are sealed together and ignited to form water. What mass of the excess reactant remains? (2H₂ + O₂ → 2H₂O)
Q7. How many electrons are present in 1.8 g of water (H₂O = 18 g/mol)?
Q8. The number of neutrons in 0.5 mol of ¹⁴₆C atoms is:
Q9. Copper has two isotopes, ⁶³Cu and ⁶⁵Cu, and its average atomic mass is 63.54. What is the percentage abundance of ⁶³Cu?
Q10. What is the approximate energy of one photon of light of wavelength 400 nm? (h = 6.63 × 10⁻³⁴ J s, c = 3 × 10⁸ m/s)
Q11. The radius of the third Bohr orbit of hydrogen is approximately (a₀ = 0.529 Å):
Q12. The energy of the electron in the n = 2 level of Li²⁺ is (Eₙ = −13.6 Z²/n² eV):
Q13. The shortest wavelength in the Balmer series (series limit) of hydrogen is closest to (R = 1.097 × 10⁷ m⁻¹):
Q14. Which spectral series of hydrogen lies entirely in the ultraviolet region?
Q15. How many orbitals and how many maximum electrons does the shell with n = 4 contain?
Q16. How many radial (spherical) nodes does a 3p orbital have?
Q17. Which set of quantum numbers (n, l, mₗ) is NOT permissible?
Q18. How many unpaired electrons are present in Fe³⁺ (atomic number 26)?
Q19. The electronic configuration of Cu⁺ (Z = 29) is:
Q20. Moseley's work on X-ray spectra showed that the square root of the frequency of a characteristic X-ray line is directly proportional to:
Q21. In Millikan's oil-drop experiment, the measured charges on different oil drops were always:
Q22. Which experiment led Chadwick to the discovery of the neutron?
Q23. Positive (canal) rays differ from cathode rays in that the e/m value of positive rays:
Q24. Which species is isoelectronic with Ar and has the nuclear charge of 17 protons?
Q25. The spin multiplicity of a nitrogen atom in its ground state (1s²2s²2p³) by Hund's rule is:
Q26. What mass of Mg(OH)₂ (58.3 g/mol) is needed to neutralise 7.3 g of HCl? (Mg(OH)₂ + 2HCl → MgCl₂ + 2H₂O)
Q27. 20 mL of methane is burned with excess oxygen at constant temperature and pressure (CH₄ + 2O₂ → CO₂ + 2H₂O, water condensed). The volume of CO₂ formed is:
Q28. 5.6 g of iron reacts with excess sulphur to give FeS (Fe = 56, S = 32). If the yield is 80%, the mass of FeS actually obtained is:
Q29. Which of the following contains the greatest number of atoms? (He = 4, H = 1, C = 12, O = 16)
Q30. Nitrogen forms oxides in which the masses of oxygen combining with 14 g of nitrogen are 8 g, 16 g, 24 g, 32 g and 40 g. This illustrates the law of: