ECAT — Engineering College Admission Test (UET)
Take the ECAT Chemistry — Stoichiometry and Atomic Structure (Set 7): 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 atoms are present in 0.50 mol of aluminium sulphate, Al₂(SO₄)₃ (Nₐ = 6.022 × 10²³ mol⁻¹)?
Answer: A. 5.12 × 10²⁴
Explanation: One formula unit has 2 + 3 + 12 = 17 atoms; 0.50 mol × 17 × 6.022 × 10²³ = 5.12 × 10²⁴.
Q2. The percentage by mass of iron in iron(III) sulphate, Fe₂(SO₄)₃ (Fe = 56, S = 32, O = 16), is:
Answer: C. 28.0%
Explanation: Molar mass = 112 + 96 + 192 = 400 g mol⁻¹; Fe = 112/400 × 100 = 28.0%.
Q3. A salt contains 32.4% Na, 22.6% S and 45.0% O by mass (Na = 23, S = 32, O = 16). Its empirical formula is:
Answer: A. Na₂SO₄
Explanation: Mole ratio = 1.409 : 0.706 : 2.81 = 2 : 1 : 4, so Na₂SO₄.
Q4. A 25.0 cm³ portion of NaOH solution is neutralised by 20.0 cm³ of 0.100 M H₂SO₄ (forming Na₂SO₄). The molarity of the NaOH solution is:
Answer: A. 0.160 M
Explanation: H₂SO₄ = 2.00 mmol, which neutralises 4.00 mmol NaOH; 4.00 mmol ÷ 25.0 cm³ = 0.160 M.
Q5. Heating 10.0 g of pure CaCO₃ strongly (CaCO₃ → CaO + CO₂; Ca = 40, C = 12, O = 16) liberates CO₂ whose volume at STP (molar volume 22.4 dm³ mol⁻¹) is:
Answer: C. 2.24 dm³
Explanation: CaCO₃ = 0.100 mol gives 0.100 mol CO₂ = 0.100 × 22.4 = 2.24 dm³.
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Q6. 10 cm³ of ethene is burned in 50 cm³ of oxygen, C₂H₄ + 3O₂ → 2CO₂ + 2H₂O. After cooling to room temperature (water condensed, same T and P) the volume of the residual gas is:
Q7. 6.0 g of magnesium is burned with 2.0 g of oxygen: 2Mg + O₂ → 2MgO (Mg = 24, O = 16). The mass of magnesium left unreacted is:
Q8. Copper has two isotopes, ⁶³Cu and ⁶⁵Cu. If the average relative atomic mass is 63.5, the percentage abundance of ⁶³Cu is:
Q9. 100 cm³ of 0.20 M NaCl solution is mixed with 200 cm³ of 0.50 M NaCl solution (volumes assumed additive). The molarity of the mixture is:
Q10. 28 g of N₂ and 8 g of H₂ are heated together in a closed reactor (N₂ + 3H₂ ⇌ 2NH₃; N = 14, H = 1) and 20.4 g of NH₃ is isolated. The percentage yield based on the limiting reagent is:
Q11. In acidic solution 2MnO₄⁻ + 5C₂O₄²⁻ + 16H⁺ → 2Mn²⁺ + 10CO₂ + 8H₂O. What volume of 0.020 M KMnO₄ oxidises 25 cm³ of 0.10 M oxalic acid?
Q12. 12.2 g of hydrated barium chloride, BaCl₂·xH₂O, gives 10.4 g of anhydrous BaCl₂ on heating (Ba = 137, Cl = 35.5, H = 1, O = 16). The value of x is:
Q13. The total number of electrons in 6.0 g of carbonate ions, CO₃²⁻ (C = 12, O = 16; Nₐ = 6.022 × 10²³ mol⁻¹), is:
Q14. Using Eₙ = −13.6 Z²/n² eV, the energy of an electron in the third orbit (n = 3) of He⁺ (Z = 2) is:
Q15. The longest-wavelength line of the Paschen series of hydrogen (n = 3 ← n = 4), with R = 1.097 × 10⁷ m⁻¹, has a wavelength of:
Q16. The ratio of the series-limit (shortest) wavelength of the Balmer series to that of the Paschen series of hydrogen is:
Q17. What is the maximum number of electrons in the n = 3 shell that can have spin quantum number s = −½?
Q18. Which of the following ions does NOT have the same number of electrons as an argon atom (Mg = 12, S = 16, Cl = 17, K = 19, Ca = 20)?
Q19. The threshold frequency of a metal is 5.0 × 10¹⁴ Hz. Light of frequency 8.0 × 10¹⁴ Hz is incident on it. Taking h = 6.626 × 10⁻³⁴ J s and e = 1.602 × 10⁻¹⁹ C, the stopping potential is:
Q20. The speed of an electron (mass 9.109 × 10⁻³¹ kg) is known to within ±1.0 × 10⁵ m s⁻¹. Taking Δx·Δp ≥ h/4π, the minimum uncertainty in its position is:
Q21. A 50 g ball moves at 20 m s⁻¹. Taking h = 6.626 × 10⁻³⁴ J s, its de Broglie wavelength is:
Q22. ²³⁸₉₂U decays to ²⁰⁶₈₂Pb by a series of α and β⁻ emissions. The number of β⁻ particles emitted is:
Q23. In the hydrogen spectrum, the ratio of the wavelength of the first line of the Lyman series to that of the first line of the Balmer series is:
Q24. In a ground-state calcium atom (Z = 20), the total number of electrons that have azimuthal quantum number l = 1 is:
Q25. A hydrocarbon of molar mass 58 g mol⁻¹ gives 1.76 g CO₂ and 0.90 g H₂O on complete combustion of 0.58 g (C = 12, H = 1, O = 16). Its molecular formula is:
Q26. A gas has density 1.964 g dm⁻³ at STP (molar volume 22.4 dm³ mol⁻¹). Its molar mass is closest to:
Q27. A solution is prepared from 46 g of ethanol (C₂H₅OH, 46 g mol⁻¹) and 54 g of water (18 g mol⁻¹). The mole fraction of ethanol is:
Q28. 6.0 dm³ of H₂ and 4.0 dm³ of O₂ (both at STP) are exploded to form liquid water. The mass of water formed (molar volume 22.4 dm³ mol⁻¹; H = 1, O = 16) is:
Q29. According to Bohr's model, the radius of the second orbit of Be³⁺ (Z = 4) compared with the first orbit of hydrogen is:
Q30. The phosphide ion ³¹₁₅P³⁻ contains how many protons, neutrons and electrons, respectively?