Elemental Composition and Compound Stoichiometry?

Question

The green pigment, chlorophyll a, absorbs red light, the energy from which powers photosynthetic processes in plants and algae.

Chlorophyll a is 6.27% nitrogen by mass.

Each molecule of chlorophyll a contains 4 nitrogen atoms.

What is the molecular mass of chlorophyll a?

Each chlorophyll a molecule contains one magnesium atom.

What mass of magnesium is to be found in 1.75 g of chlorophyll a?

could someone please help me out, Thanks!

Answer 1

mass of 4 N atoms = 14.007*4 = 56.028

This is 6.27% of the chlorophyll molar mass

Molar mass = (100/6.27)*56.028 = 893.6g/mole

893.6 g chlorophyll contains 24.305g Mg

1.75g chlorophyll will contain: 24.305/893.6*1.75 = 0.0476g Mg

1.75g chlorophyl will contain 0.0476g or 47.6mg Mg

Answer 2

This is the best solution i can give at the moment.  I looked up the actual MM of chlorophyll and its 892.9g/mol, compared to my answer of 837g/mole.  That is however close enough to know that i probably just made a small mistake (its like 1 in the morning lol) and i bet if you started over using this method you will get the right answer, its just that I'm out of time and gota go, I hope this helpes.  I just did the first part.

let mass of chlorophyll = 100g

therefore mass of nitrogen = 6.27g

and mass of chlorophyll = 93.73g

(6.27g of N) / (14.007g/mole)   *grams cancel

= 0.4476 mole of N

N to Chlorophyll ration = 4:1

(0.4476 mole of N) x  [ (1 mole of Chlorophyll) / (4 mole of N) ] *nitrogen's cancel leaving just Chlorophyll

0.4476 x (1/4) = 0.112 mole of chlorophyll

(93.73g) / (0.122 mole of chlorophyll)  = 837g/mole