Let us suppose that we have 1 mol of FeCr2O4. I'm going to use approximate masses because I have no idea what your periodic table will say. Just put in the exact masses from your periodic table.
Fe = 56 Cr*2 = 2*52 = 104 O4 = 4*16 = 64 =========== Total = 56 + 104 + 64 = 224 The % oxygen = (64 / 224) * 100 = 28.5 % but none of your answers match this. Perhaps you are talking about Fe2(Cr2O4)3 The brackets make all the difference in the world.
Without going through all the detail I did before, The molecular mass is Fe * 2 = 112 Cr * 6 = 312 O * 12 = 192 The total molecular mass = 616
The % Oxygen = (192 / 616) * 100 = 31% roughly. That answer isn't there either.
When sugar is dissolved in water, the solution does not conduct electricity, because there are no ions in the solution. Some substances that are made of molecules form solutions that do conduct electricity. Ammonia is such a substance.
