Question

Chlorophyll, the green colouring matter of plants responsible for photosynthesis, contains 2.68% of magnesium by mass. Calculate the number of magnesium atoms in 2.00 g of chlorophyll. 

Answer 1

100 g chlorophyll contains 2.68 g Mg = 2.68/24 mole Mg

2 g chlorophyll contains 2.68 x 2/24 x 100 mole Mg = 2.2 x 10^-3 mole Mg

No. of Mg atoms = 2.2 x 10^-3 x 6.023 x 10²³

= 1.345 x 10²¹ atoms of Mg