Why is molality preferred over molarity for colligative properties?

Molality is mol solute per kg solvent — a mass ratio independent of temperature. Molarity uses solution volume, which expands when heated, changing the value.

How is Avogadro's number defined today?

Since the 2019 SI redefinition, the Avogadro constant is fixed exactly at 6.02214076×10²³ mol⁻¹; the mole is defined to contain that number of entities.

Is STP molar volume always 22.4 L?

22.4 L/mol applies at the older STP (0 °C, 1 atm). IUPAC's newer STP (0 °C, 100 kPa) gives 22.71 L/mol; MDCAT still uses 22.4 L.

What does a base peak in mass spectrometry indicate?

It is the most intense peak, set to 100% relative abundance. It is not necessarily the molecular ion — fragmentation can make a daughter ion the most stable.

How many oxygen atoms are present in 0.25 mol of CO₂?

3.0×10²³. 0.25 mol CO₂ × 2 O per molecule × 6.022×10²³ ≈ 3.0×10²³ atoms.

A hydrocarbon contains 85.7% C and 14.3% H by mass. Its empirical formula is:

CH₂. Moles: C = 85.7/12 = 7.14; H = 14.3/1 = 14.3. Ratio 1:2 → CH₂.

What mass of NaOH (M = 40) is needed to make 500 mL of 0.20 M solution?

4.0 g. n = MV = 0.20 × 0.500 = 0.10 mol; mass = 0.10 × 40 = 4.0 g.

For the reaction N₂ + 3 H₂ → 2 NH₃, mixing 1 mol N₂ with 2 mol H₂ produces at most:

1.33 mol NH₃. H₂ is limiting: 2 mol H₂ × (2 mol NH₃ / 3 mol H₂) = 1.33 mol NH₃.

The volume of 0.05 mol of an ideal gas at STP is approximately:

1.12 L. V = n × 22.4 = 0.05 × 22.4 = 1.12 L at STP.

Fundamental Concepts of Chemistry MDCAT 2026 — Notes, Key Concepts & MCQs (Chemistry)

Q: What is the difference between empirical and molecular formula?

The empirical formula gives the smallest whole-number ratio of atoms; the molecular formula gives the actual number. Glucose has empirical CH₂O and molecular C₆H₁₂O₆.

Fundamental Concepts of Chemistry

Fundamental Concepts averages 2 MCQs per MDCAT paper — mole calculations, empirical formulae, and stoichiometry of limiting reagents dominate.

Fundamental Concepts of Chemistry is a Chemistry chapter on the official PMDC MDCAT 2026 syllabus, contributing roughly 2 MCQs to the 45-MCQ Chemistry section. Mastering the core concepts below typically secures the full chapter weightage.

The mole, Avogadro's number, and amount of substance

One mole is exactly 6.02214076×10²³ elementary entities — the Avogadro constant N_A. Mass of one mole in grams equals the relative atomic or molecular mass: 1 mol of H₂O is 18.015 g, 1 mol of CO₂ is 44.01 g. The MDCAT loves the chain n = m/M = N/N_A = V/22.4 (for an ideal gas at STP). A 9.0 g sample of water therefore contains 0.5 mol = 3.011×10²³ molecules and 1.5×10²⁴ atoms (because each H₂O has three atoms). Atkins' Physical Chemistry Chapter 1 stresses that the mole is a counting unit, not a mass — a habit the FSc Punjab Textbook Chemistry XI Chapter 1 reinforces with worked examples.

Empirical and molecular formulae

The empirical formula gives the simplest whole-number ratio of atoms; the molecular formula gives the actual count. The standard recipe: take the percentage composition as grams, divide each by the atomic mass to get moles, divide all by the smallest, and scale to whole numbers. A compound 40.0% C, 6.7% H, 53.3% O gives moles 3.33:6.7:3.33 → 1:2:1 → CH₂O (empirical). If the molar mass is 180 g/mol, the molecular formula is (CH₂O)₆ = C₆H₁₂O₆ — glucose. A frequent trap is rounding 1.33 to 1; multiply by 3 to get 4 and keep the ratio honest.

Stoichiometry and the limiting reagent

For 2 H₂ + O₂ → 2 H₂O, mixing 4 mol H₂ with 1 mol O₂ means O₂ is limiting (you would need 2 mol O₂ to consume all H₂). The limiting reagent fixes the maximum (theoretical) yield; percentage yield = (actual / theoretical) × 100. If 2 mol H₂O is the theoretical yield and only 1.7 mol forms, percentage yield is 85%. Always convert grams to moles before comparing — comparing masses directly is the single most common MDCAT error in this chapter.

Concentration, molarity, and the parts-per scales

Molarity M = mol solute / L solution; molality m = mol solute / kg solvent. To prepare 250 mL of 0.10 M NaOH (M_r = 40), dissolve 0.025 mol × 40 = 1.0 g and dilute to the mark. Dilution rule M₁V₁ = M₂V₂: diluting 50 mL of 2.0 M HCl to 500 mL gives 0.20 M. Mass percent = (mass solute / mass solution) × 100, ppm = (mass solute / mass solution) × 10⁶. Note molality is preferred for colligative-property work because it does not vary with temperature, unlike molarity.

Isotopes, average atomic mass, and the mass spectrometer

Isotopes are atoms of the same element with different neutron numbers, e.g. ¹²C and ¹³C. Average atomic mass = Σ (isotopic mass × fractional abundance). Chlorine's value of 35.45 comes from 75.77% ³⁵Cl + 24.23% ³⁷Cl. Mass spectrometry separates isotopes by m/z after ionisation, acceleration through a potential, and deflection in a magnetic field — the canonical instrument cited in both Atkins and FSc Chapter 1. A spectrum's tallest peak (base peak) is not always the molecular ion; questions exploit this difference.

Key Concepts

Worked MCQs

Frequently Asked Questions

How Fundamental Concepts of Chemistry Is Tested

MDCAT questions on Fundamental Concepts of Chemistry are a mix of recall (definitions, classifications), application (predict outcomes, interpret diagrams), and basic numerical/analytical reasoning. PMDC papers from 2020–2025 emphasized the concepts above; older UHS papers (2008–2019) tested them too, with slight variations in question framing.