ECAT — Engineering College Admission Test (UET)
Take the ECAT Chemistry — Stoichiometry and Atomic Structure (Set 5): 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 hydrogen atoms are present in 8.5 g of ammonia, NH₃? (N = 14, H = 1; Nₐ = 6.022 × 10²³ mol⁻¹)
Answer: A. 9.033 × 10²³
Explanation: 8.5 g ÷ 17 g mol⁻¹ = 0.50 mol NH₃ = 1.50 mol H atoms = 1.50 × 6.022 × 10²³ = 9.033 × 10²³.
Q2. Which of these nitrogen fertilizers has the highest mass percentage of nitrogen? (H = 1, C = 12, N = 14, O = 16, S = 32, Cl = 35.5)
Answer: D. Urea, CO(NH₂)₂
Explanation: %N: urea 28/60 = 46.7 %; NH₄NO₃ 28/80 = 35.0 %; NH₄Cl 14/53.5 = 26.2 %; (NH₄)₂SO₄ 28/132 = 21.2 %.
Q3. A 10.0 g mixture of CaCO₃ and CaO is strongly heated until no more gas is evolved (CaCO₃ → CaO + CO₂). The mass loss is 2.2 g. What is the mass percentage of CaCO₃ in the original mixture? (Ca = 40, C = 12, O = 16)
Answer: B. 50 %
Explanation: Loss = CO₂ = 2.2 g = 0.05 mol, so CaCO₃ = 0.05 mol × 100 = 5.0 g. 5.0/10.0 = 50 %.
Q4. A hypothetical element has two isotopes, of mass numbers 69 and 71, and its average relative atomic mass is 69.8. What is the percentage abundance of the heavier isotope? (Take isotopic masses equal to mass numbers.)
Answer: B. 40 %
Explanation: Let the heavier fraction be x: 69(1 − x) + 71x = 69.8 → 2x = 0.8 → x = 0.40 = 40 %.
Q5. 10.8 g of aluminium reacts with 21.3 g of chlorine gas: 2Al + 3Cl₂ → 2AlCl₃. What mass of AlCl₃ is formed? (Al = 27, Cl = 35.5)
Answer: A. 26.7 g
Explanation: Al = 0.40 mol (needs 0.60 mol Cl₂); Cl₂ = 0.30 mol only, so Cl₂ is limiting. 0.30 × (2/3) = 0.20 mol AlCl₃ × 133.5 = 26.7 g.
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Q6. What volume of oxygen, measured at STP (molar volume 22.4 dm³ mol⁻¹), is needed for complete combustion of 4.4 g of propane, C₃H₈? (C = 12, H = 1)
Q7. 30 cm³ of ethane, C₂H₆, is burned in 150 cm³ of oxygen (all volumes at the same temperature and pressure). After the reaction the mixture is cooled to the original temperature so that water is liquid. What is the total volume of gas remaining?
Q8. Which of the following contains the greatest total number of atoms? (H = 1, C = 12, N = 14, O = 16)
Q9. Taking the mass of an α-particle as four times that of a proton and its charge as twice that of a proton, the ratio of the charge-to-mass ratio of a proton to that of an α-particle is
Q10. Chlorine consists of ³⁵Cl and ³⁷Cl in the abundance ratio 3 : 1. In the mass spectrum of Cl₂ molecules, the peaks at m/e = 70, 72 and 74 have relative intensities of about
Q11. What is the radius of the third Bohr orbit of Li²⁺ (Z = 3)? Take the radius of the first Bohr orbit of hydrogen as 0.529 Å.
Q12. Using Eₙ = −13.6 Z²/n² eV, the energy of the electron in the n = 2 level of Li²⁺ (Z = 3) is
Q13. The ratio of the longest wavelength in the Lyman series to the longest wavelength in the Balmer series of hydrogen is
Q14. A sample of hydrogen atoms has all of its electrons excited to n = 5, and they then return to lower levels by every possible route. How many different spectral lines are emitted that belong to the Lyman or Balmer series?
Q15. An electron is accelerated from rest through a potential difference of 150 V. Its de Broglie wavelength is closest to (h = 6.63 × 10⁻³⁴ J s, mₑ = 9.11 × 10⁻³¹ kg, e = 1.60 × 10⁻¹⁹ C; neglect relativity)
Q16. What is the maximum number of electrons in an atom that can have n = 4 and mₗ = +2?
Q17. How many unpaired electrons are there in a free gaseous Co³⁺ ion in its ground state? (Z of Co = 27)
Q18. In the ground state of an argon atom, how many electrons have mₗ = 0?
Q19. Copper (Z = 29) has the ground-state configuration [Ar]3d¹⁰4s¹. How many electrons in a copper atom occupy s orbitals altogether?
Q20. In Moseley's law √ν = a(Z − b) with b = 1 for Kα lines, copper (Z = 29) gives Kα wavelength 1.54 Å. The predicted Kα wavelength of nickel (Z = 28) is
Q21. Approximately how many photons of wavelength 500 nm are needed to carry a total energy of 1.0 J? (h = 6.63 × 10⁻³⁴ J s, c = 3.0 × 10⁸ m s⁻¹)
Q22. The uncertainty in the speed of an electron is 1.0 × 10⁵ m s⁻¹. Using Δx·Δp ≥ h/4π, the minimum uncertainty in its position is nearest to (h = 6.63 × 10⁻³⁴ J s, mₑ = 9.11 × 10⁻³¹ kg)
Q23. In the thermite reaction Fe₂O₃ + 2Al → Al₂O₃ + 2Fe, 48 g of Fe₂O₃ is heated with 10.8 g of Al. Which statement is correct? (Fe = 56, Al = 27, O = 16)
Q24. Nitrogen forms the oxides N₂O, NO, N₂O₃ and NO₂. The mass of oxygen that combines with a fixed 28 g of nitrogen in N₂O₃ compared with that in N₂O is in the ratio
Q25. Which pair of nuclides are isobars (same mass number, different element)? (Z: Ar = 18, K = 19, Ca = 20)
Q26. How many neutrons are present in 2.0 g of heavy water, D₂O? (D = ²H, O = ¹⁶O; Nₐ = 6.0 × 10²³)
Q27. In Chadwick's 1932 experiment, α-particles bombarded beryllium-9 and an uncharged radiation was detected. Which nuclear equation represents the reaction?
Q28. In an atom with more than one electron, which order of subshell energies within the third shell is correct?
Q29. An oxide of vanadium contains 56.0 % V by mass. What is its empirical formula? (V = 51, O = 16)
Q30. Heating 50.0 g of pure CaCO₃ gives 21.0 g of CaO. What is the percentage yield? (Ca = 40, C = 12, O = 16)