ECAT — Engineering College Admission Test (UET)
Take the ECAT Chemistry — Stoichiometry and Atomic Structure (Set 2): 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 molecules are present in 4.4 g of CO₂ (C = 12, O = 16)?
Answer: A. 6.022 × 10²²
Explanation: Moles = 4.4/44 = 0.1 mol; molecules = 0.1 × 6.022 × 10²³ = 6.022 × 10²².
Q2. The percentage of oxygen by mass in H₂SO₄ is about:
Answer: D. 65.3%
Explanation: M(H₂SO₄) = 98; mass of O = 4 × 16 = 64; 64/98 × 100 = 65.3%.
Q3. What volume is occupied by 11 g of CO₂ at STP?
Answer: A. 5.6 dm³
Explanation: 11/44 = 0.25 mol; 0.25 × 22.4 = 5.6 dm³.
Q4. 2.4 g of Mg is added to a solution containing 0.1 mol of HCl (Mg + 2HCl → MgCl₂ + H₂). The volume of H₂ evolved at STP is:
Answer: D. 1.12 dm³
Explanation: Mg = 0.1 mol needs 0.2 mol HCl, so HCl (0.1 mol) is limiting; it gives 0.05 mol H₂ = 0.05 × 22.4 = 1.12 dm³.
Q5. 10 g of H₂ and 64 g of O₂ are sparked together to form water. What mass of the excess reactant remains unreacted?
Answer: C. 2 g of H₂
Explanation: O₂ = 2 mol needs 4 mol (8 g) H₂; H₂ supplied is 10 g, so 10 − 8 = 2 g of H₂ remains.
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Q6. 14 g of N₂ reacts with excess H₂ (N₂ + 3H₂ → 2NH₃) and 8.5 g of NH₃ is obtained. The percentage yield is:
Q7. A hydrocarbon contains 85.7% C and 14.3% H by mass, and its molar mass is 56 g mol⁻¹. Its molecular formula is:
Q8. How many ions are produced when 0.1 mol of Al₂(SO₄)₃ dissolves completely in water?
Q9. Magnesium consists of ²⁴Mg (79%), ²⁵Mg (10%) and ²⁶Mg (11%). Its average atomic mass is about:
Q10. In a mass spectrometer operated at a fixed electric and magnetic field, which ion is deflected the most?
Q11. The numbers of neutrons and electrons in the ion ³⁷Cl⁻ (Z = 17) are respectively:
Q12. Using Bohr's theory, the energy of the electron in the first orbit of Li²⁺ (Z = 3) is (energy of H in n = 1 is −13.6 eV):
Q13. The minimum energy needed to remove an electron from the n = 2 orbit of a hydrogen atom is:
Q14. In hydrogen, the second line of the Balmer series arises from the n = 4 → n = 2 transition. Its wavelength is about (R = 1.097 × 10⁷ m⁻¹):
Q15. Using rₙ = 0.529 n²/Z Å, the radius of the second Bohr orbit of He⁺ (Z = 2) is:
Q16. An electron is accelerated from rest through a potential difference of 150 V. Its de Broglie wavelength is about (h = 6.63 × 10⁻³⁴ J s, m = 9.11 × 10⁻³¹ kg, e = 1.6 × 10⁻¹⁹ C):
Q17. Heisenberg's uncertainty principle implies that:
Q18. The total number of orbitals in the shell with principal quantum number n = 3 is:
Q19. For an electron in a 4d orbital, the possible values of the magnetic quantum number m are:
Q20. Which ion has the electronic configuration [Ar] 3d¹⁰?
Q21. The number of unpaired electrons in Fe³⁺ (Z of Fe = 26) is:
Q22. An element has the valence-shell configuration 4s² 4p³. Its position in the periodic table is:
Q23. Light with photon energy 5.0 eV falls on a metal whose work function is 2.0 eV. The maximum kinetic energy of the emitted photoelectrons is:
Q24. The frequency of a photon carrying 3.3 × 10⁻¹⁹ J of energy is about (h = 6.63 × 10⁻³⁴ J s):
Q25. In Rutherford's gold-foil experiment, the observation that most α-particles passed straight through the foil showed that:
Q26. The neutron was discovered by Chadwick when:
Q27. What volume of O₂ measured at STP is needed for the complete combustion of 16 g of methane (CH₄ + 2O₂ → CO₂ + 2H₂O)?
Q28. What mass of KClO₃ (M = 122.5) must be decomposed to give 3.36 dm³ of O₂ at STP (2KClO₃ → 2KCl + 3O₂)?
Q29. Which sample contains the same number of molecules as 16 g of O₂?
Q30. A p orbital has how many nodal planes passing through the nucleus?