NUMS — Medical & Dental Entry Test
Take the NUMS Chemistry — Fundamental Concepts, Atomic Structure and Bonding (Set 6): 30 multiple-choice questions (40 minutes) with instant scoring and a…
What this test covers
The first 5 questions of this test, with the correct answer and explanation.
Q1. Boron has two stable isotopes, ¹⁰B (isotopic mass 10.013) and ¹¹B (isotopic mass 11.009). The relative atomic mass of natural boron is 10.81. What is the percentage abundance of ¹⁰B, to three significant figures?
Answer: B. 20.0%
Explanation: Let the fraction of ¹⁰B be x: 10.013x + 11.009(1 − x) = 10.81, so x = (11.009 − 10.81)/(11.009 − 10.013) = 0.200, i.e. 20.0%. The value 80.0% is the abundance of ¹¹B.
Q2. In a mass spectrometer, chlorine gas (Cl₂) gives molecular-ion peaks at m/z 70, 72 and 74 from the isotopic combinations ³⁵Cl–³⁵Cl, ³⁵Cl–³⁷Cl and ³⁷Cl–³⁷Cl. Taking the abundance ratio ³⁵Cl : ³⁷Cl as exactly 3 : 1, what is the relative intensity of the peaks at m/z 70 : 72 : 74?
Answer: D. 9 : 6 : 1
Explanation: Probabilities with p(³⁵Cl) = 3/4 and p(³⁷Cl) = 1/4: m/z 70 = (3/4)² = 9/16; m/z 72 = 2 × (3/4)(1/4) = 6/16 (two arrangements); m/z 74 = (1/4)² = 1/16. Ratio 9 : 6 : 1.
Q3. What is the mass of one mole of electrons? (mass of an electron = 9.109 × 10⁻³¹ kg, Nₐ = 6.022 × 10²³ mol⁻¹)
Answer: C. 5.49 × 10⁻⁴ g
Explanation: Mass = 9.109 × 10⁻³¹ kg × 6.022 × 10²³ = 5.49 × 10⁻⁷ kg = 5.49 × 10⁻⁴ g. (The wrong option 5.49 × 10⁻⁷ is the figure in kg.)
Q4. 5.4 g of aluminium is added to a solution containing 7.3 g of hydrogen chloride: 2Al + 6HCl → 2AlCl₃ + 3H₂ (Al = 27, HCl = 36.5). What volume of hydrogen, at STP (22.4 dm³ mol⁻¹), is liberated?
Answer: A. 2.24 dm³
Explanation: Al = 0.20 mol (needs 0.60 mol HCl for complete reaction) but HCl = 0.20 mol only, so HCl is limiting. 6 mol HCl gives 3 mol H₂, so 0.20 mol HCl gives 0.10 mol H₂ = 2.24 dm³. Using Al as the basis would give 6.72 dm³.
Q5. In a blast furnace, 160 kg of pure Fe₂O₃ is reduced: Fe₂O₃ + 3CO → 2Fe + 3CO₂ (Fe = 56, O = 16). If 98 kg of iron is actually obtained, what is the percentage yield?
Answer: D. 87.5%
Explanation: Theoretical yield: 160 kg Fe₂O₃ = 1000 mol, giving 2000 mol Fe = 112 kg. Percentage yield = 98/112 × 100 = 87.5%. (61.3% results from wrongly dividing by 160.)
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Q6. A sodium hydroxide solution is 20% NaOH by mass (NaOH = 40 g mol⁻¹, remainder water). What is its molality?
Q7. A 12.3 g sample of hydrated magnesium sulphate, MgSO₄·xH₂O, leaves 6.0 g of anhydrous MgSO₄ (M = 120 g mol⁻¹) on strong heating. What is the value of x? (H₂O = 18 g mol⁻¹)
Q8. A green laser pointer emits light of wavelength 532 nm at a power of 5.0 mW. About how many photons does it emit per second? (h = 6.626 × 10⁻³⁴ J s, c = 3.00 × 10⁸ m s⁻¹)
Q9. Taking R_H = 1.097 × 10⁷ m⁻¹, what is the wavelength of the line emitted by a He⁺ ion in the transition n = 3 → n = 2? (1/λ = R_H Z²(1/n₁² − 1/n₂²))
Q10. How many electrons in a ground-state vanadium atom (Z = 23) have the azimuthal quantum number l = 1?
Q11. Which of these ions, each in its free gaseous state, has the largest spin-only magnetic moment? (Z: Cr = 24, Mn = 25, Fe = 26, Ni = 28)
Q12. Consider NO⁺, NO and NO⁻ (valence-electron counts 10, 11 and 12). Which statement follows from molecular orbital theory?
Q13. In the Lewis structure of the sulphate ion drawn with four S–O single bonds so that every atom obeys the octet rule, what is the formal charge on sulphur? (formal charge = valence electrons − non-bonding electrons − number of bonds)
Q14. Which of these species is NOT trigonal planar?
Q15. What are the number of lone pairs on the central atom and the molecular shape of IF₅?
Q16. Which of these compounds has a zero net dipole moment?
Q17. Which ranking of boiling points, from highest to lowest, is correct for ethanol, ethylamine, dimethyl ether and propane (all of similar molar mass)?
Q18. The H–P–H bond angle in phosphine (PH₃) is about 93°, whereas the H–N–H angle in ammonia is about 107°. What is the best explanation?
Q19. How many σ bonds and π bonds are present in a molecule of methylbenzene (toluene), C₆H₅CH₃, using the Kekulé structure of the ring?
Q20. What is the hybridisation of the nitrogen atom in the nitrate ion (NO₃⁻) and in the nitrite ion (NO₂⁻), respectively?
Q21. Use the Born–Haber data for potassium chloride (kJ mol⁻¹): ΔH_f° (KCl) = −437; atomisation of K(s) = +89; first ionisation energy of K = +419; ½Cl₂(g) → Cl(g) = +122; electron affinity of Cl = −349. The lattice enthalpy is defined here as the enthalpy change when gaseous ions form one mole of solid KCl. What is its value?
Q22. For an ionic solid, the coordination number depends on the radius ratio r(cation)/r(anion): 0.225–0.414 gives 4, 0.414–0.732 gives 6 and 0.732–1.0 gives 8. Zn²⁺ has radius 74 pm and S²⁻ has radius 184 pm. What coordination number is predicted for Zn²⁺ in ZnS?
Q23. A He⁺ ion emits a line in the transition n = 4 → n = 2. This line has exactly the same wavelength as which transition in a hydrogen atom?
Q24. 20 cm³ of carbon monoxide is mixed with 20 cm³ of oxygen and the mixture is sparked: 2CO + O₂ → 2CO₂. What is the total volume of gas after the reaction, when measured at the original temperature and pressure and with any water absent?
Q25. A compound contains 26.6% potassium, 35.4% chromium and 38.1% oxygen by mass (K = 39.1, Cr = 52.0, O = 16.0). What is its formula?
Q26. How many moles of ions (cations plus anions) are present in 250 cm³ of 0.20 M aluminium sulphate, Al₂(SO₄)₃, assuming complete dissociation?
Q27. Which sample contains the greatest total number of atoms? (He = 4, H = 1, N = 14, O = 16)
Q28. The Lyman-α line of hydrogen has a wavelength of 121.6 nm. What is the energy of one mole of photons of this wavelength? (h = 6.626 × 10⁻³⁴ J s, c = 3.00 × 10⁸ m s⁻¹, Nₐ = 6.022 × 10²³ mol⁻¹)
Q29. How many radial nodes and angular nodes does a 5p orbital have?
Q30. Which pair of nuclides are isotones (same number of neutrons)?