How do you calculate the percent composition of each element in K2Cr2O7?

You must calculate the weight of each element and of the molecule

The mass of the elemenst are bellow : K: 39 Cr:52 O:16

So, the total mass of 1 mol of the molecule is: 39.(2)+52.(2)+16.(7) Total mass= 294 g/mol

So, 294 g is 100%, and if u want to find the respective element composition, u just need to devide the mass of 1 mol of the element in the molecule for the molecule mass:

Composition of K= (2x39)/ 294 = 0,26 = 26,5% Composition of Cr= (2x52)/294 = 35,4% Composition of O = (16x7) /294 = 38,1%

If u do the sum, it must be equal 100%