Question

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Vitamin B12, cyanocobalamin, contains 4.34% cobalt by mass. Calculate the molar mass, assuming that there is one atom of cobalt in every molecule of cyanocobalamin.

Answer 1

Okay. I think this is how you're supposed to do it...

You already have the mass of colbat in one mole of vitamin B12. You also know that vitamin B12 is 4.34% of the total mass. So in order to find the total mass of the vitamin, you just have to keep scaling up till you reach the total 100%.

You can calculate how much one atom of colbat (Cobalt is 58.93g.) weighs by using this: 58.93/ 6.02 X 1023 (6.02 X 1023 is how much one mole of any substance contains.)
Multiply the answer by 6.02 X 1023 so you can get the number of molecules in one mole of cyanocobalamin. Once done, you will get the amount of colbat (in grams) and 1 mole of cyanocobalamin (which according to your question is 4.34% of its mass).

All of this sounds confusing, but it really is just a simple (or not) ratio:

The molar mass of vitamin B12/58.93 is equivalent to: 100%/4.34%
Therefore to discover the molar mass: 58.93*100/4.34 = 58.93/0.0434


I read this answer from maybe 6-7 years ago, and its stuck in my brain and I don't even know why. I'm going to figure out where I read it, maybe the source will help you answer your question.

Hope I helped!

Answer 2

OKay, so I found the site where I read this years ago:
http://www.chemicalforums.com/index.php?topic=15716.0

Answer 3

I also found another site right now! Just trying to give you some resources:
https://answers.yahoo.com/question/index?qid=20090726133921AAFUWCt