Q4. Chlorine has two isotopes, Cl and Cl. Which of Dalton’s postulates does this contradict, and why?
Postulate 2, that all atoms of an element have identical mass. Cl and Cl are atoms of the same element, chlorine, with the very similar chemical properties but different masses (about 35 u and 37 u).
Q5. Using Dalton’s postulates, explain why 18.016 g of pure water from any source always contains 2.016 g of hydrogen (H = 1.008 u, O = 16.00 u).
Every water molecule has exactly 2 H atoms and 1 O atom (postulate 3), and every H atom and every O atom has its own fixed mass (postulate 2). So each molecule is 2.016 parts hydrogen in 18.016 by mass, and so is any amount of water: 18.016 g contains 2.016 g of hydrogen.
Q6. Sulphur forms SO2 and SO3. Find the mass of oxygen combined with 32.06 g of sulphur in each (O = 16.00 u), and the ratio.
SO2: 2 × 16.00 = 32.00 g. SO3: 3 × 16.00 = 48.00 g. Ratio 32.00 : 48.00 = 2 : 3.