ECAT — Engineering College Admission Test (UET)
Take the ECAT Chemistry — Stoichiometry and Atomic Structure (Set 6): 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. A sample of 2.86 g of washing soda, Na₂CO₃·xH₂O, loses all its water on strong heating and leaves 1.06 g of anhydrous Na₂CO₃ (Na = 23, C = 12, O = 16, H = 1). The value of x is:
Answer: D. 10
Explanation: Water lost = 2.86 − 1.06 = 1.80 g = 0.100 mol; Na₂CO₃ = 1.06 ÷ 106 = 0.0100 mol. x = 0.100 ÷ 0.0100 = 10.
Q2. 5.4 g of aluminium is heated with 14.2 g of chlorine gas: 2Al + 3Cl₂ → 2AlCl₃ (Al = 27, Cl = 35.5). The mass of AlCl₃ formed is:
Answer: D. 17.8 g
Explanation: Al = 0.200 mol, Cl₂ = 0.200 mol. Cl₂ needs 0.300 mol for 0.200 mol Al, so Cl₂ is limiting: 0.200 × 2/3 = 0.1333 mol AlCl₃ × 133.5 = 17.8 g.
Q3. In acidic solution MnO₄⁻ is reduced to Mn²⁺ while Fe²⁺ is oxidised to Fe³⁺. What volume of 0.20 M KMnO₄ is needed to oxidise 20 cm³ of 0.50 M FeSO₄?
Answer: B. 10 cm³
Explanation: 5Fe²⁺ + MnO₄⁻ + 8H⁺ → 5Fe³⁺ + Mn²⁺ + 4H₂O. Fe²⁺ = 10 mmol, so MnO₄⁻ = 2 mmol; volume = 2 mmol ÷ 0.20 M = 10 cm³.
Q4. 14 g of nitrogen reacts with excess hydrogen, N₂ + 3H₂ → 2NH₃, but only 50% of the nitrogen is converted. The mass of ammonia obtained is (N = 14, H = 1):
Answer: D. 8.5 g
Explanation: 14 g N₂ = 0.50 mol → theoretical 1.0 mol NH₃; at 50% yield, 0.50 mol × 17 g mol⁻¹ = 8.5 g.
Q5. Chlorine consists of ³⁵Cl (75%) and ³⁷Cl (25%). In the mass spectrum of Cl₂ molecules, the relative intensities of the peaks at m/z = 70, 72 and 74 are in the ratio:
Answer: B. 9 : 6 : 1
Explanation: Combinations: ³⁵Cl₂ = 0.75² = 9/16; ³⁵Cl³⁷Cl = 2(0.75)(0.25) = 6/16; ³⁷Cl₂ = 0.25² = 1/16. Ratio 9 : 6 : 1.
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Q6. The number of neutrons in 4.0 g of heavy water, D₂O (D = ²H, O = ¹⁶O; molar mass 20 g mol⁻¹; Nₐ = 6.022 × 10²³ mol⁻¹) is:
Q7. Complete combustion of 0.46 g of a compound containing only C, H and O gives 0.88 g CO₂ and 0.54 g H₂O. Its empirical formula is (C = 12, H = 1, O = 16):
Q8. 15 cm³ of propane is burned in 100 cm³ of oxygen, C₃H₈ + 5O₂ → 3CO₂ + 4H₂O, and the products are cooled to room temperature so that water is liquid. The total volume of gas then remaining (same T and P) is:
Q9. A concentrated acid is 49% H₂SO₄ by mass and has density 1.40 g cm⁻³ (H₂SO₄ = 98 g mol⁻¹). Its molarity is:
Q10. Magnesium has three isotopes: ²⁴Mg (79%), ²⁵Mg (10%) and ²⁶Mg (11%). Its average relative atomic mass is:
Q11. What is the percentage by mass of water in copper(II) sulphate pentahydrate, CuSO₄·5H₂O (Cu = 63.5, S = 32, O = 16, H = 1)?
Q12. The mass of a single molecule of glucose, C₆H₁₂O₆ (molar mass 180 g mol⁻¹), is:
Q13. Aluminium-27 bombarded with α-particles produces a neutron and a new nucleus: ²⁷₁₃Al + ⁴₂He → X + ¹₀n. The nucleus X is:
Q14. Using Bohr's expression rₙ = 0.529 n²/Z Å, the radius of the second orbit of Li²⁺ (Z = 3) is:
Q15. An electron in He⁺ (Z = 2) falls from n = 2 to n = 1. Using 1/λ = RZ²(1/n₁² − 1/n₂²) with R = 1.097 × 10⁷ m⁻¹, the wavelength emitted is:
Q16. The shortest wavelength (series limit) in the Balmer series of hydrogen, taking R = 1.097 × 10⁷ m⁻¹, is:
Q17. A hydrogen sample is excited so that its electrons reach n = 6, and then all possible downward transitions occur. How many of the emitted lines belong to the Balmer series (ending at n = 2)?
Q18. A proton and an electron are accelerated so that they have the same kinetic energy. Taking mₚ/mₑ = 1836, the ratio of the electron's de Broglie wavelength to the proton's is about:
Q19. The ionisation energy of hydrogen from its ground state is 13.6 eV. The energy needed to remove the electron from Li²⁺ when it is in its ground state (n = 1) is:
Q20. How many different orbitals with magnetic quantum number m = +2 can exist for electrons with principal quantum number n ≤ 4?
Q21. Which of the following ions has the greatest number of unpaired electrons in its ground state (Cr = 24, Mn = 25, Ni = 28, Zn = 30)?
Q22. An orbital with two radial nodes and one angular node must be:
Q23. Consider ³⁹₁₉K, ⁴⁰₁₈Ar and ⁴⁰₂₀Ca. Which pair are isotones (same number of neutrons)?
Q24. A laser of power 1.00 W emits light of wavelength 663 nm. Taking h = 6.626 × 10⁻³⁴ J s and c = 3.00 × 10⁸ m s⁻¹, the number of photons emitted per second is:
Q25. A metal has a threshold wavelength of 300 nm. Light of wavelength 200 nm falls on it. Taking hc = 1240 eV nm, the maximum kinetic energy of the ejected electrons is:
Q26. A mixture of CO and CO₂ occupying 11.2 dm³ at STP has a mass of 18 g (C = 12, O = 16; molar volume 22.4 dm³ mol⁻¹). The mass of CO in the mixture is:
Q27. 10.0 g of impure limestone reacts with excess HCl and evolves 2.016 dm³ of CO₂ at STP (CaCO₃ = 100 g mol⁻¹; molar volume 22.4 dm³ mol⁻¹). Assuming the impurities do not react, the percentage purity of CaCO₃ is:
Q28. What volume of 0.25 M H₃PO₄ is needed to neutralise 30 cm³ of 0.50 M NaOH completely to Na₃PO₄?
Q29. The number of neutrons present in 1.4 g of carbon-14 (¹⁴₆C, assume molar mass 14 g mol⁻¹; Nₐ = 6.022 × 10²³ mol⁻¹) is:
Q30. A 2.0 g sample of an iron oxide contains 1.4 g of iron (Fe = 56, O = 16). The formula of the oxide is: