Some Basic Concepts of Chemistry — Important Questions

Equal number of molecules

Step 1: Avogadro's Hypothesis (1811) states that under identical conditions of temperature and pressure, equal volumes of all gases contain equal number of molecules. Step 2: The key word is 'molecules', not atoms — because different gases have different atomicities (e.g., He is monoatomic, O₂ is diatomic, O₃ is triatomic). Step 3: Equal atoms would be wrong because 1 L of He has 1 atom per molecule while 1 L of O₂ has 2 atoms per molecule — so atom count would differ. Step 4: Equal masses is wrong because different gases have different molar masses (e.g., H₂ = 2 g/mol, O₂ = 32 g/mol). Step 5:

C₆H₁₂O₆

Step 1: First calculate the empirical formula mass. CH₂O → 12 + 2(1) + 16 = 30 g/mol. Step 2: Find n = Molecular mass / Empirical formula mass = 180 / 30 = 6. Step 3: Molecular formula = n × empirical formula = 6 × CH₂O = C₆H₁₂O₆. Step 4: C₆H₁₂O₆ is glucose — a very important biological molecule! This confirms the formula. Step 5: C₃H₆O₃ would give n = 3 (molecular mass = 90), and C₂H₄O₂ gives n = 2 (molecular mass = 60). Neither matches 180. The correct answer is C₆H₁₂O₆.

Law of Conservation of Mass

Step 1: The Law of Conservation of Mass was given by Antoine Lavoisier (1774). It states that in any physical or chemical change, matter is neither created nor destroyed — it just changes form. Step 2: This is why we balance chemical equations — the total mass of reactants must equal the total mass of products. Step 3: Example: CaCO₃ (100 g) → CaO (56 g) + CO₂ (44 g). Total products = 56 + 44 = 100 g = mass of reactant. Step 4: Law of Constant Proportion (Proust) deals with fixed ratios in a compound. Law of Multiple Proportions (Dalton) deals with two elements forming multiple compounds. Step

3.011 × 10²³

Step 1: The mole concept states that 1 mole of any substance contains 6.022 × 10²³ molecules (Avogadro's number). Step 2: Number of molecules = Number of moles × Avogadro's number. Step 3: Number of molecules = 0.5 × 6.022 × 10²³ = 3.011 × 10²³. Step 4: 6.022 × 10²³ would be the answer for 1 mole, not 0.5 mole — a common error. Step 5: 1.204 × 10²⁴ = 2 × 6.022 × 10²³, which would be 2 moles. Always multiply moles by Avogadro's number to get number of molecules.