I noticed something in JGK's post. He has a Oxygen MW at 15.9994. With 4 molecues in CaSO4 it should be factored as 63.9976 (X4) which in my calculations would give me the factor of 4.246.
True, but as your original post did not say you were quantifying Oxygen to determine [CaSO4], I didnt proceed any further than givong the MW of oxygen and stating that it formed 47% of CaSO4.
Plus if you are using the oxygen to calculate [CaSO4], the factor you use depand on how you characterise the oxygen in the analysis.
If you measure [ O ] then [ O ]/0.47 = [CaSO4]
If you measure [ O2 ] then [ O2 ]/(0.47/2) = [CaSO4]