Stoichiometry...can someone explain this?

Question

"Fungal laccase, a blue protein found in wood-rotting fungi, is 0.390% Cu by mass. If a fungal laccase molecule contains 4 copper atoms, what is the molar mass of fungal laccase?"

The answer is 65100g, but I don't understand how the teacher the reasoning used to derived this answer...can someone explain it for me?

Answer 1

look on the periodic table for the atomic mass of copper. this is how much one mole of pure copper will weigh in grams. multiply by four to get how much the four molecules in the fungal laccase weigh. this weight is 0.390% of the total molar weight of the molecule. you want 100% of the weight so divide 100 by 0.390 to get the number you need to multiply the weight of copper by. Now multiply the weight of the four copper atoms by the number you got and hopefully you should get the right answer.

I just worked that off the top of my head and I havn't done it with a periodic table so do the calculation and see if you get the right answer.....

Answer 2

Let's break down the % to make sense of this:

0.390% copper by mass is the same as (0.390g copper/100g total mass of laccase)*100

Cu is 63.55g/mole, but you only want the mass of 4 atoms. Using 63.55g Cu = 6.022x 10exp23 atoms of Cu, we can calculate how much 4 atoms of Cu will weigh.....4.22 x 10exp-22g. This means that 1 molecule of laccase will contain 4.22 x 10exp-22g copper.

Now I would use a ratio to solve for MM of laccase:

4.22 x 10exp-22g Cu times X = .390g Cu

Solve for X to calculate how many molecules of laccase in 100g of laccase. X= 9.24 x 10exp20 molecules laccase in 100g laccase.

6.022 x 10exp23 molecules laccase*(100g lacase/9.24 x 10exp20 molecules laccase) = 65179g/1 mole

I have rounded sig figs for each step, which is why my number is higher.

Hope that helps!