masa molara carbonat de potasiu (K2CO3) = 2x39 + 12 + 3x16 = 138g/mol
in 138 g....................78g potasiu....................12gcarbon
6,9g K2CO3 .........x.......................................y
x = 6,9x78/138 = 3,9g potasiu
y= 6,9x12/138 = 0,6g carbon