percentage mass question ( gcse chem)

Find the % mass of oxygen in CaCO3 when:
Ca=20
C=6
O=16
I did this question and got an answer of 64.9%,but the textbook says the answer is 48%.
my working out is:
20+6+(16x3)+74
48/74=0.649 (rounded up)
0.649x100=64.9%
i cannot work out what i am doing wrong, or if it is just a typo. If i am wrong, please tell me why! :woo:

Reply 1

Olibert

You're using the atomic numbers as far as I can see. You want to be using the relative atomic masses.

The atomic numbers only counts the number of protons in the atom. The relative atomic mass includes protons and neutrons.

Reply 2

Norby

mcgoohan

carbon is mass 12. Atomic number is 6
Calcium has a mass of 40. Atomic number is 20

You then get 40+12+48=100
48/100=0.48=48%

Reply 3

mcgoohan

thanks both of you, that is very helpful :yes:

No problemo!

Percentage mass of an element in a compound = ((atomic mass × number of atoms of that element) ÷ formula mass of the comound) × 100

Ar of Ca = 40
Arof C = 12
Arof O = 16

Mrof CaCO3 = 40 12 (16×3) = 100

((16×3) ÷ 100) × 100=48

(edited 6 years ago)