Atoms and Molecules — Important Questions

18 g

Step 1: To find the molecular mass of H₂O, we add the atomic masses of all atoms present in the molecule. Step 2: H₂O contains 2 hydrogen atoms and 1 oxygen atom. Step 3: Mass of 2 hydrogen atoms = 2 × 1 = 2 amu. Step 4: Mass of 1 oxygen atom = 1 × 16 = 16 amu. Step 5: Total molecular mass = 2 + 16 = 18 g. So the gram molecular mass of water is 18 g. Option A (16 g) counts only oxygen. Option B (17 g) adds only one hydrogen. Option D (20 g) is incorrect arithmetic.

Equal number of molecules

Step 1: Avogadro's Law was proposed by Italian scientist Amedeo Avogadro in 1811. Step 2: The law states that equal volumes of all gases at the same temperature and pressure contain equal NUMBER OF MOLECULES. Step 3: For example, 1 litre of hydrogen and 1 litre of oxygen at the same temperature and pressure both contain the same number of molecules. Step 4: The masses of gases are NOT equal because different molecules have different masses. Step 5: This is why gases with different densities can still contain the same number of molecules in equal volumes. Option A is wrong because molecules hav

Molecular Mass = 2 × Vapour Density

Step 1: Vapour Density (V.D.) is the ratio of the mass of a given volume of a gas to the mass of an equal volume of hydrogen at the same conditions. Step 2: According to Avogadro's law, equal volumes contain equal number of molecules. Step 3: So V.D. = mass of 1 molecule of gas / mass of 2 atoms of hydrogen (because hydrogen is diatomic, H₂). Step 4: Relative Molecular Mass = mass of 1 molecule / mass of 1 atom of hydrogen. Step 5: Comparing both formulas: Molecular Mass = 2 × Vapour Density. For example, if V.D. of CO₂ is 22, then molecular mass = 44. This is a key formula to remember.

2 moles

Step 1: The formula to calculate the number of moles from mass is: Number of moles = Mass / Atomic Mass. Step 2: Here, Mass of sodium = 46 g and Atomic mass of sodium (Na) = 23. Step 3: Number of moles = 46 / 23 = 2 moles. Step 4: This means 46 g of sodium contains 2 × 6.023 × 10²³ atoms of sodium. Step 5: Option A (1 mole) would be correct for 23 g of Na. Option C (0.5 mole) would be correct for 11.5 g of Na. Always divide the given mass by the atomic mass to find moles.