Some Basic Concepts of Chemistry — Important Questions

Dalton believed atoms of the same element could not combine with each other, so he rejected diatomic molecules like H₂ and O₂

Step 1 - Recall Dalton's atomic theory: atoms are indivisible and atoms of the same element are identical; compounds form when atoms of DIFFERENT elements combine. Step 2 - The key flaw: Dalton believed same-type atoms CANNOT combine. This means he rejected the existence of H₂ and O₂ as diatomic molecules. Step 3 - Without diatomic molecules, the volume ratios cannot be explained. For example, if O is a single atom, you cannot get 2 water molecules from 1 oxygen. Step 4 - Avogadro later correctly proposed diatomic molecules (H₂, O₂) and his law of equal volumes containing equal molecules expla

0.4 M

Step 1 - Write the formula: Molarity = moles of solute / volume of solution in litres. Step 2 - Calculate moles of NaOH: moles = mass / molar mass = 4 g / 40 g/mol = 0.1 mol. Step 3 - Convert volume to litres: 250 mL = 250/1000 = 0.25 L. Step 4 - Calculate molarity: M = 0.1 mol / 0.25 L = 0.4 mol/L = 0.4 M. Step 5 - Check: 0.1 M is wrong because it forgets to divide by 0.25 (not 1 L). 1.6 M multiplies instead of divides. 0.04 M divides moles by 2.5 instead of 0.25, a decimal point error.

Carbon dioxide (CO₂) and carbon monoxide (CO): masses of oxygen combining with 12g of carbon are 32g and 16g — ratio 2:1

Step 1 - Recall Law of Multiple Proportions (Dalton, 1803): When two elements form more than one compound, masses of one element combining with a fixed mass of the other are in a simple whole number ratio. Step 2 - In CO: 12g C combines with 16g O. In CO₂: 12g C combines with 32g O. Step 3 - Ratio of oxygen masses = 16:32 = 1:2, a simple whole number ratio. This perfectly illustrates the law. Step 4 - Option B describes the Law of Definite Proportions (Proust), not multiple proportions. Step 5 - Option C describes Avogadro's Law. Option D describes Law of Conservation of Mass (Lavoisier). Each

75%

Step 1 - Let the fractional abundance of ³⁵Cl = x. Then abundance of ³⁷Cl = (1 - x). Step 2 - Set up the weighted average equation: 34.97x + 36.97(1 - x) = 35.5. Step 3 - Expand: 34.97x + 36.97 - 36.97x = 35.5. Simplify: -2x = 35.5 - 36.97 = -1.47. Step 4 - Solve: x = 1.47/2 = 0.735 ≈ 0.75. Step 5 - % abundance of ³⁵Cl = 75%. This makes physical sense because the average (35.5) is much closer to 35 than to 37, indicating ³⁵Cl is more abundant. 50% would give average = 35.97, not 35.5. 25% is the abundance of ³⁷Cl.