NUMS — Medical & Dental Entry Test

NUMS Chemistry — Fundamental Concepts, Atomic Structure and Bonding (Set 5)

Take the NUMS Chemistry — Fundamental Concepts, Atomic Structure and Bonding (Set 5): 30 multiple-choice questions (40 minutes) with instant scoring and a…

What this test covers

• 30 NUMS Chemistry

Sample questions with answers

The first 5 questions of this test, with the correct answer and explanation.

1. Q1. A 0.500 g sample containing chloride is dissolved and treated with excess AgNO₃, giving 1.435 g of dry AgCl (M = 143.5 g mol⁻¹). What is the percentage of chlorine (Cl = 35.5) by mass in the sample?

   • A. 35.5%
   • B. 71.0% ✓
   • C. 28.7%
   • D. 14.2%

   Answer: B. 71.0%

   Explanation: n(AgCl) = 1.435/143.5 = 0.0100 mol, so 0.0100 mol Cl = 0.355 g. Percentage = 0.355/0.500 × 100 = 71.0%.

2. Q2. How many hydrogen atoms are present in 1.8 g of water? (H₂O, M = 18 g mol⁻¹; N_A = 6.02 × 10²³ mol⁻¹)

   • A. 3.01 × 10²³
   • B. 1.20 × 10²³ ✓
   • C. 1.80 × 10²⁴
   • D. 6.02 × 10²²

   Answer: B. 1.20 × 10²³

   Explanation: n(H₂O) = 1.8/18 = 0.10 mol; each molecule has 2 H atoms, so 0.20 mol H = 0.20 × 6.02 × 10²³ = 1.20 × 10²³ atoms.

3. Q3. How many oxygen atoms are present in 0.25 mol of the hydrated salt Al₂(SO₄)₃·18H₂O? (N_A = 6.02 × 10²³ mol⁻¹)

   • A. 1.81 × 10²⁴
   • B. 7.22 × 10²⁴
   • C. 4.52 × 10²⁴ ✓
   • D. 2.89 × 10²⁴

   Answer: C. 4.52 × 10²⁴

   Explanation: One formula unit has 12 O in 3 SO₄²⁻ plus 18 O in the water of crystallisation = 30 O. 0.25 × 30 = 7.5 mol O × 6.02 × 10²³ = 4.52 × 10²⁴.

4. Q4. Magnesium (7.29 g, Mg = 24.3) is burnt completely in nitrogen: 3Mg + N₂ → Mg₃N₂. The nitride is then fully hydrolysed: Mg₃N₂ + 6H₂O → 3Mg(OH)₂ + 2NH₃. What volume of NH₃ is released at STP? (molar volume 22.4 L mol⁻¹)

   • A. 1.49 L
   • B. 6.72 L
   • C. 2.24 L
   • D. 4.48 L ✓

   Answer: D. 4.48 L

   Explanation: n(Mg) = 7.29/24.3 = 0.300 mol → 0.100 mol Mg₃N₂ → 0.200 mol NH₃. V = 0.200 × 22.4 = 4.48 L.

5. Q5. The WHO guideline for fluoride in drinking water is 1.5 ppm (taken as 1.5 mg of F⁻ per litre of solution). What is this as a molarity? (F = 19 g mol⁻¹)

   • A. 1.5 × 10⁻³ M
   • B. 2.9 × 10⁻² M
   • C. 7.9 × 10⁻⁵ M ✓
   • D. 7.9 × 10⁻² M

   Answer: C. 7.9 × 10⁻⁵ M

   Explanation: 1.5 mg = 1.5 × 10⁻³ g; moles = 1.5 × 10⁻³/19 = 7.9 × 10⁻⁵ mol in 1 L, so 7.9 × 10⁻⁵ M.

The other 25 questions

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6. Q6. Concentrated sulphuric acid is 98% H₂SO₄ by mass with density 1.84 g mL⁻¹ (M = 98.08 g mol⁻¹). What is its molarity?

   • A. 1.84 M
   • B. 18.4 M
   • C. 36.8 M
   • D. 9.2 M

7. Q7. How many electrons are present in 1.8 g of ammonium ions, NH₄⁺? (M = 18 g mol⁻¹; N_A = 6.02 × 10²³ mol⁻¹)

   • A. 5.42 × 10²³
   • B. 6.02 × 10²²
   • C. 6.62 × 10²³
   • D. 6.02 × 10²³

8. Q8. The Paschen series of hydrogen consists of transitions to n = 3. Using R_H = 1.097 × 10⁷ m⁻¹, what is the wavelength of the series limit (n = ∞ → 3) and in which region does it lie?

   • A. 820 nm, infrared
   • B. 365 nm, ultraviolet
   • C. 1875 nm, infrared
   • D. 656 nm, visible

9. Q9. According to Bohr's theory, rₙ = 0.529 n²/Z Å. What is the radius of the third orbit of He⁺ (Z = 2)?

   • A. 1.06 Å
   • B. 1.59 Å
   • C. 4.76 Å
   • D. 2.38 Å

10. Q10. An electron is accelerated from rest through a potential difference of 150 V. Using λ = h/√(2meV) (≈ 1.226/√V nm with V in volts), what is its de Broglie wavelength?

    • A. 0.0100 nm
    • B. 1.00 nm
    • C. 0.100 nm
    • D. 0.150 nm

11. Q11. What is the longest wavelength of light that can ionise a hydrogen atom that is already in the n = 2 level? (Eₙ = −13.6/n² eV; hc = 1240 eV nm)

    • A. 365 nm
    • B. 91 nm
    • C. 656 nm
    • D. 122 nm

12. Q12. Using Slater's rules (an electron in the same ns,np group shields 0.35; each electron in the n − 1 shell shields 0.85; each electron in shells n − 2 or lower shields 1.00), what is the effective nuclear charge on the 4s electron of potassium (Z = 19)?

    • A. 3.20
    • B. 2.20
    • C. 1.00
    • D. 8.20

13. Q13. In the ground state of a chromium atom (Z = 24), how many electrons have the azimuthal quantum number l = 2?

    • A. 6
    • B. 5
    • C. 4
    • D. 10

14. Q14. For which of these elements is the difference between the second and first ionisation energies the largest?

    • A. Silicon
    • B. Magnesium
    • C. Aluminium
    • D. Sodium

15. Q15. Which species has the shortest bond length?

    • A. O₂⁺
    • B. O₂⁻
    • C. O₂²⁻
    • D. O₂

16. Q16. How many lone pairs are on the central iodine atom of the triiodide ion, I₃⁻, and what is its shape?

    • A. Two; linear
    • B. Two; bent
    • C. Three; linear
    • D. Three; bent

17. Q17. What are the hybridisation states of the central xenon atom in XeF₂ and XeF₄ respectively?

    • A. sp³d² and sp³d²
    • B. sp³ and sp³d
    • C. sp³d and sp³d²
    • D. sp³d and sp³d

18. Q18. HCl has a measured dipole moment of 1.03 D and a bond length of 1.27 Å. Taking the dipole of a pure unit-charge separation as 4.80 D per Å × length, what is the percentage ionic character?

    • A. 21.5%
    • B. 84.3%
    • C. 8.2%
    • D. 16.9%

19. Q19. Which compound is expected to have the highest melting point?

    • A. NaF
    • B. KBr
    • C. MgO
    • D. NaCl

20. Q20. Tetracyanoethylene, (NC)₂C=C(CN)₂, has the molecular formula C₆N₄. How many σ and π bonds does it contain?

    • A. 9 σ and 9 π
    • B. 9 σ and 8 π
    • C. 8 σ and 10 π
    • D. 10 σ and 8 π

21. Q21. n-Pentane boils at 36 °C whereas its isomer 2,2-dimethylpropane (neopentane) boils at 9.5 °C. What is the best explanation?

    • A. n-Pentane has a permanent dipole moment but neopentane has none
    • B. Linear molecules make larger surface contact, so London forces are stronger
    • C. n-Pentane forms hydrogen bonds between molecules but neopentane does not
    • D. Neopentane has a higher molar mass, so its molecules escape more easily

22. Q22. What is the mass in grams of a single sodium atom? (Na = 22.99 g mol⁻¹; N_A = 6.022 × 10²³ mol⁻¹)

    • A. 1.66 × 10⁻²⁴ g
    • B. 6.02 × 10⁻²³ g
    • C. 3.82 × 10⁻²³ g
    • D. 2.29 × 10⁻²² g

23. Q23. 50.0 mL of 0.200 M BaCl₂ is mixed with 150.0 mL of 0.100 M Na₂SO₄. Ignoring any solubility of BaSO₄ (M = 233.4 g mol⁻¹), what mass of precipitate forms and what is [SO₄²⁻] left in the final 200 mL?

    • A. 3.50 g; 0.100 M
    • B. 2.33 g; 0.075 M
    • C. 3.50 g; 0.025 M
    • D. 2.33 g; 0.025 M

24. Q24. A metal M forms an oxide of formula M₂O₃. When 1.35 g of the metal is heated in oxygen it gives 2.55 g of oxide. What is the atomic mass of M? (O = 16)

    • A. 27
    • B. 54
    • C. 40.5
    • D. 13.5

25. Q25. How many neutrons are present in 1.0 mol of carbon-14 atoms? (N_A = 6.02 × 10²³ mol⁻¹)

    • A. 3.61 × 10²⁴
    • B. 6.02 × 10²³
    • C. 4.82 × 10²⁴
    • D. 8.43 × 10²⁴

26. Q26. In an oil-drop experiment, the charges measured on three drops were 4.8 × 10⁻¹⁹ C, 8.0 × 10⁻¹⁹ C and 11.2 × 10⁻¹⁹ C. What is the largest value of the elementary charge consistent with all three results?

    • A. 4.8 × 10⁻¹⁹ C
    • B. 1.6 × 10⁻¹⁹ C
    • C. 3.2 × 10⁻¹⁹ C
    • D. 0.8 × 10⁻¹⁹ C

27. Q27. How many electrons in the n = 3 shell of an atom can have ms = +½? (Assume the shell is completely filled.)

    • A. 6
    • B. 3
    • C. 9
    • D. 18

28. Q28. A large sample of hydrogen atoms is excited so that all are in the n = 5 level. How many distinct spectral lines can appear as they return to lower levels by all possible routes?

    • A. 10
    • B. 4
    • C. 5
    • D. 20

29. Q29. Which nitrogenous fertiliser contains the highest percentage of nitrogen by mass? (N = 14.0, H = 1.0, C = 12.0, O = 16.0, S = 32.0, Cl = 35.5)

    • A. Ammonium sulphate, (NH₄)₂SO₄
    • B. Urea, CO(NH₂)₂
    • C. Ammonium nitrate, NH₄NO₃
    • D. Ammonium chloride, NH₄Cl

30. Q30. 200 mL of 0.50 M HCl is mixed with 300 mL of 1.00 M HCl. Assuming volumes are additive, what is the molarity of the mixture?

    • A. 0.70 M
    • B. 1.50 M
    • C. 0.75 M
    • D. 0.80 M