Chlorophyll contains 2.68% of magnesium by mass.Calculate the number of magnesium atoms in 3.00 g of chlorophyll

a) 1.98 * 10²¹ atoms b) 6.023 * 10²³ atoms c) 1.7 * 10²⁰ atoms d) 2.8 * 10^{2 2} atoms

We are given that chlorophyll contains 2.68 % of chlorophyll. Therefore mass of magnesium that is present in 3g of chlorophyll is 

 (2.68 / 100) X 3 = 0.0804g

Thus 3g of chlorophyll contains 0.0804g of Mg. Now the molar mass of magnesium is 24.312g. This means that there are 6.023 X 10²³ atoms of magnesium present in 24.312g of Mg. So

 24.312 g of Mg = 6.023 X 10²³ atoms of Mg

 or 1 g of Mg = (6.023 X 10²³/ 24.312) atoms of Mg

 Hence 0.0804 g of Mg = (6.023 X 10²³ X 0.0804) / 24.312 atoms of Mg

 = 1.992 X 10²¹ atoms of Magnesium. 

The option that is closest to the answer is a) 1.98 X 10²¹ atoms