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Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: sundrops on January 21, 2005, 02:04:37 AM

Title: Elemental Composition and Compund Stoichiometry
Post by: sundrops on January 21, 2005, 02:04:37 AM

The green pigment chlorophyll a, absorbs red light, the energy from which powers photosynthetis 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?

I'm really confused, I don't even know how tio attack this question - I mean I don't know what chlorophyll is made up of - it doesn't seem like there's enough info. Please *delete me*

Title: Re:Elemental Composition and Compund Stoichiometry
Post by: Mitch on January 21, 2005, 02:38:15 AM

What is the mass of 4 Nitrogens?

Title: Re:Elemental Composition and Compund Stoichiometry
Post by: sundrops on January 21, 2005, 02:45:12 AM

mass of 4 nitrogens would be 14.01 * 4 = 56.04g
right? but how does that help me?

Title: Re:Elemental Composition and Compund Stoichiometry
Post by: Mitch on January 21, 2005, 02:52:18 AM

That was the mass of 6.27% of the molecule.
What is the mass of the other 93.73% of the molecule? Then calculate the total mass for 1 molecule.

Title: Re:Elemental Composition and Compund Stoichiometry
Post by: sundrops on January 21, 2005, 03:00:11 AM

OH! that makes sense! but the mass of the rest of the rest of the molecule isn't given! thats what makes this question sooo hard! not enough information. its frusterating!!

Title: Re:Elemental Composition and Compund Stoichiometry
Post by: sundrops on January 21, 2005, 03:34:19 AM

how do i find the mass of the other 93.73% of the molecule?

Title: Re:Elemental Composition and Compund Stoichiometry
Post by: AWK on January 21, 2005, 06:55:42 AM

6.27/56.04 = 100/x
where x is the molecular mass of chlorophyl

Title: Re:Elemental Composition and Compund Stoichiometry
Post by: sundrops on January 21, 2005, 10:18:07 AM

oh - that makes sense  :D
thanks so much!

Title: Re:Elemental Composition and Compund Stoichiometry
Post by: sundrops on January 21, 2005, 02:42:01 PM

now what if the chlorophyll molecule contains 1 magnesium atom.
what mass of magnesium is to be found in 2.97 g of chlorophyll?

Title: Re:Elemental Composition and Compund Stoichiometry
Post by: sundrops on January 21, 2005, 03:10:22 PM

how would i go about trying to solve that?

Title: Re:Elemental Composition and Compund Stoichiometry
Post by: Mitch on January 21, 2005, 03:47:56 PM

It wouldn't matter.

Title: Re:Elemental Composition and Compund Stoichiometry
Post by: sundrops on January 21, 2005, 05:00:55 PM

but that's part II of the question - i need to figure it out somehow. I need to find out how much magnesium is in 2.97 g of chlorophyll - how would i go about doing that?

Title: Re:Elemental Composition and Compund Stoichiometry
Post by: dexangeles on January 21, 2005, 05:36:15 PM

you already have the total mass of chlorophyl, so the mass of 1 magnesium atom is what percent of that total mass?

start from there  ;)

Title: Re:Elemental Composition and Compund Stoichiometry
Post by: sundrops on January 22, 2005, 05:37:17 AM

ok so if percent nitrogen is 6.27% and the mass of nitrrogen is 56.04g

then with the mass of 1 magnesium atom being 24.31g - then it's percentage of the total mass would be 2.72% !! ;D

that makes sense!

Title: Re:Elemental Composition and Compund Stoichiometry
Post by: sundrops on January 22, 2005, 05:39:24 AM

now, how would I apply that information into determinig how much magnesium I would find in 2.97g of chlorophyll?  :-\

Title: Re:Elemental Composition and Compund Stoichiometry
Post by: dexangeles on January 22, 2005, 01:14:07 PM

check your givens and write them down

2.97 grams of chlorophyll

900 g/mol chlorophyll

24.31 g/mol of magnesium

percent of magnesium


as far as i remember, you can either use percent calculation or follow stoichiometry

Title: Re:Elemental Composition and Compund Stoichiometry
Post by: sundrops on January 22, 2005, 11:00:36 PM

thanks for everyones input - i finally figured it out!  ;D  (finally!) ouch - my head really hurts now... ::)