ECAT — Engineering College Admission Test (UET)
Take the ECAT Chemistry — Stoichiometry and Atomic Structure (Set 4): 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 0.60 g sample of an organic compound containing only C, H and O gives 0.88 g CO₂ and 0.36 g H₂O on complete combustion. What is its empirical formula?
Answer: C. CH₂O
Explanation: C = 0.88×12/44 = 0.24 g (0.02 mol); H = 0.36×2/18 = 0.04 g (0.04 mol); O = 0.60 − 0.28 = 0.32 g (0.02 mol). Ratio 1 : 2 : 1 gives CH₂O.
Q2. A 2.00 g mixture of CaCO₃ and inert sand loses 0.44 g in mass when strongly heated until constant mass. What is the percentage of CaCO₃ in the mixture? (Ca = 40, C = 12, O = 16)
Answer: C. 50%
Explanation: Mass lost is CO₂: 0.44/44 = 0.01 mol, so 0.01 mol CaCO₃ = 1.00 g. Percentage = 1.00/2.00 × 100 = 50%.
Q3. In the reaction 4NH₃ + 5O₂ → 4NO + 6H₂O, 3.4 g of NH₃ is mixed with 4.0 g of O₂. What mass of NO can form at most? (N = 14, H = 1, O = 16)
Answer: D. 3.0 g
Explanation: NH₃ = 0.2 mol needs 0.25 mol O₂, but only 0.125 mol O₂ is present, so O₂ limits. NO = 0.125 × 4/5 = 0.10 mol = 3.0 g.
Q4. A drop of water has a mass of 0.09 g. Approximately how many hydrogen atoms does it contain? (H = 1, O = 16, Nₐ = 6.02×10²³)
Answer: B. 6.0×10²¹
Explanation: 0.09 g / 18 g mol⁻¹ = 0.005 mol of H₂O. Each molecule has 2 H atoms, so 0.010 mol × 6.02×10²³ ≈ 6.0×10²¹.
Q5. An element X has two isotopes, ⁶³X and ⁶⁵X, and its average relative atomic mass is 63.5. What is the percentage abundance of ⁶³X?
Answer: C. 75%
Explanation: Let abundance of ⁶³X be x: 63x + 65(1 − x) = 63.5 gives 65 − 2x = 63.5, so x = 0.75, i.e. 75%.
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Q6. In a mass spectrometer all ions enter the same magnetic field with the same speed. Which ion is deflected through the path of smallest radius (most strongly)?
Q7. How does the radius of the third Bohr orbit of He⁺ compare with the radius of the first Bohr orbit of hydrogen?
Q8. What is the energy of the electron in the second Bohr orbit of Li²⁺? (Use Eₙ = −13.6 Z²/n² eV)
Q9. What is the ratio (longest wavelength of the Lyman series) : (series-limit wavelength of the Balmer series) in the hydrogen spectrum?
Q10. An electron and a proton have the same kinetic energy. The ratio of de Broglie wavelengths λ(electron) / λ(proton) is closest to (mass of proton ≈ 1836 × mass of electron)
Q11. How many radial (spherical) nodes does a 4p orbital have?
Q12. What is the maximum number of electrons in one atom that can have n = 3 and mₗ = +1?
Q13. Which of the following is NOT isoelectronic with argon?
Q14. What is the ground-state electronic configuration of the Cr²⁺ ion? (Z for Cr = 24)
Q15. The Kα X-ray frequency of an element is 4 times that of sodium (Z = 11). Using Moseley's law √ν = a(Z − b) with b = 1, the atomic number of the element is
Q16. J. J. Thomson found the same e/m value for cathode-ray particles whatever the gas or the cathode metal used. What did this indicate?
Q17. A hydrated salt MgSO₄·xH₂O contains 51.2% water by mass. Find x. (Mg = 24, S = 32, O = 16, H = 1)
Q18. 10 mL of a gaseous hydrocarbon is burned in excess oxygen and gives 40 mL of CO₂ and 50 mL of water vapour, all measured at the same temperature and pressure. The hydrocarbon is
Q19. Sulphur (32 g) is burnt to SO₂ with 90% yield, and all the SO₂ obtained is then converted to SO₃ with 80% yield. What mass of SO₃ is obtained? (S = 32, O = 16)
Q20. Which sample contains the largest number of atoms? (H = 1, C = 12, N = 14, O = 16)
Q21. How many protons, neutrons and electrons are present in ⁵⁶₂₆Fe³⁺?
Q22. Which set of quantum numbers (n, l, mₗ, mₛ) is permitted for an electron in an atom?
Q23. The first line of the Balmer series (n = 3 → 2) of hydrogen is calculated with R = 1.097×10⁷ m⁻¹. Its wavelength is about
Q24. In the Bohr model, the speed of the electron is proportional to Z/n. How does the speed of the electron in the second orbit of He⁺ compare with that in the first orbit of hydrogen?
Q25. The mass of a single atom of an element is 1.99×10⁻²³ g. Its molar mass is about (Nₐ = 6.02×10²³)
Q26. 50 mL of 0.10 M BaCl₂ is mixed with 30 mL of 0.10 M Na₂SO₄. Assuming complete precipitation of BaSO₄, what is the concentration of Ba²⁺ left in the 80 mL of solution?
Q27. Positive (canal) rays from different gases are compared. Which gas gives positive rays with the highest e/m ratio?
Q28. 6.5 g of a divalent metal M reacts with excess dilute acid to give 2.24 L of H₂ at STP (22.4 L mol⁻¹). The atomic mass of M is
Q29. The speed of an electron is uncertain by 1×10⁵ m s⁻¹. Using Δx·Δp ≥ h/4π, the minimum uncertainty in its position is about (h = 6.63×10⁻³⁴ J s, mₑ = 9.11×10⁻³¹ kg)
Q30. The ground-state nitrogen atom (1s² 2s² 2p³) has its three 2p electrons in separate orbitals with parallel spins. Its total spin quantum number S (sum of mₛ values) is