Q4. A copper ore is 12.0% chalcopyrite, CuFeS2, by mass; the rest is gangue with no copper. How much copper does 1.00 tonne of the ore hold? (Cu = 63.55, Fe = 55.85, S = 32.06 g mol−1.)
M(CuFeS2) = 63.55 + 55.85 + 2 × 32.06 = 183.52 g mol−1, so CuFeS2 is 63.55/183.52 × 100 = 34.6% copper. The tonne holds 120 kg of CuFeS2, and so 120 × 0.3463 = 41.6 kg of copper (reasoned).
Q5. A pyrolusite ore is 80.0% MnO2 by mass. What mass of manganese is there in 500 kg of it? (Mn = 54.94, O = 16.00 g mol−1.)
M(MnO2) = 54.94 + 32.00 = 86.94 g mol−1, so MnO2 is 54.94/86.94 = 0.6319 manganese by mass. 500 kg of ore holds 400 kg of MnO2, and so 400 × 0.6319 = 253 kg of manganese (reasoned).
Q6. Why are the ores of so many metals oxides?
Oxygen is a very reactive element and is very abundant on the earth, so over time many metals have ended up combined with it (NCERT Class 10 Science, Chapter 3, page 50).
Q7. Assertion: iron is not found native in large deposits. Reason: iron is a moderately reactive metal. Are both true, and does the reason explain the assertion?
Both are true, and the reason explains the assertion: metals in the middle of the activity series, such as iron, occur mainly as oxides, sulphides or carbonates (NCERT Class 10 Science, Chapter 3, page 50).
Q8. Find the percentage of iron in pure siderite, FeCO3, and the mass of siderite that holds as much iron as 1.00 tonne of pure magnetite, Fe3O4. (Fe = 55.85, C = 12.01, O = 16.00 g mol−1.)
M(FeCO3) = 55.85 + 12.01 + 48.00 = 115.86 g mol−1, so siderite is 55.85/115.86 × 100 = 48.2% iron. Magnetite is 72.4% iron (Example 1), so 1.00 tonne of it holds 0.7236 t of iron. The siderite needed is 0.7236/0.4820 = 1.50 t (reasoned).