Calculate the percentage by mass of the indicated element in the following compounds.

Question

Carbon in acetylene, C2H2 , a gas used in welding.
Express your answer using three significant figures.

Hydrogen in ascorbic acid, HC6H7O6 , also known as vitamin C .
Express your answer using two significant figures.

Hydrogen in ammonium sulfate, (NH4)2SO4 , a substance used as a nitrogen fertilizer.
Express your answer using two significant figures.

Platinum in PtCl2(NH3)2 , a chemotherapy agent called cisplatin.
Express your answer using four significant figures.

Oxygen in the female sex hormone estradiol, C18H24O2 .
Express your answer using three significant figures.

Carbon in capsaicin, C18H27NO3 , the compound that gives the hot taste to chili peppers.
Express your answer using three significant figures.

Answer 1

The percentage by mass is determined by 1. multiplying each elemental mass by the number of atoms in a molecule, 2. adding the result, and then dividing the individual masses by the total mass.

 

1. acetylene has the empirical formula C₂H₂,  the elemental mass of Carbon is 12, and Hydrogen is 1. So (12x2) + (1x2) = 26. We then divide the elemental masses by the molecular mass. So, 24 / 26 = 92.308% for Carbon

 

2. ascorbic acid has the empirical formula C₆H₈O₆, the elemental mass of Carbon is 12, Hydrogen is 1, and Oxygen is 16. So, (12x6) + (1x8) + (16x6) = 176. So 8/176 = 4.54% for Hydrogen

 

The other problems will be solved in the same way.