Din cele 7 grame de amestec, 5% sunt impuritati
100 g amestec .............5 g impuritati
7 grame amestec .......x impuritati
x = 7 x 5 : 100 = 0,35 grame impuritati
masa fier + masa sulf = masa totala - masa impuritati = 7 g - 0,35 g = 6,65g
masa sulf = 2 masa fier
masa fier + masa sulf = masa fier + 2 masa fier = 6,65 g
3 masa fier = 6,65 g
masa fier = 6,65 : 3 = 2,2166 g
masa sulf = 2 x 2,2166 g = 4,433 g