How do I find the number of moles?

Question

Hi,

I am wondering how to find the number of moles of a certain element in "X" amount (grams) of an amino acid.

The amino acid that was given to us is valine (C₅H₁₁NO₂). The website wants us to provide it with the number of moles of carbon in 98.92 grams of valine.

I've read the chapter but still don't understand how I would do this.

Thanks!

Answer 1

First calculate the molar mass of valine. Carbon has a molar mass of 12.01g/mol, hydrogen has a molar mass of 1.01g/mol, nitrogen has a molar mass of 14.01g/mol and oxygen has a molar mass of 16g/mol. There are 5 carbon atoms, 11 hydrogen atoms, 1 nitrogen atom and 2 oxygen atoms in valine, so valine's molar mass is 5 x 12.01 + 11 x 1.01 + 1 x 14.01 + 2 x 16 = 117.17g/mol.

We have 98.92g of valine so find the number of moles of valine using dimensional analysis.

98.92g valine x 1 mol valine/117.17g valine = 0.8442 mol valine.

Lastly to find the number of moles of carbon, note that in this mole of valine, there are five moles of carbon, due to carbon's subscript. So multiply the 0.8442 moles of valine by 5 to get the moles of carbon.