how to calculate the atomic masses using relative relative atomic carbon
Answers
Find the relative mass of any atom by adding the number of protons to the number of neutrons. Hydrogen has a relative atomic mass of 1, and carbon-12 has a relative atomic mass of 12. Isotopes of the same element have different numbers of neutrons, so you need to calculate for one specific isotope.
Explanation:
METHOD 1
To find the average atomic mass, you take a certain number of atoms, find the total mass of each isotope, and then divide the total mass of all the atoms by the total number of atoms.
Assume that you have, say, 10 000 atoms of carbon. Then you have 9893 atoms of
12
C
and 107 atoms of
13
C
.
Mass of
l
12
C
=
9893
atoms
×
12 u
1
atom
=
118 716 u
Mass of
l
13
C
=
107
atoms
×
13.003 u
1
atom
=
m
l
1 390 u
Total mass
=
120 110 u
Average mass
=
120 110 u
10 000 atoms
=
12.011 u
METHOD 2
Another way of determining the average mass is to multiply the atomic mass of each isotope by its percentage and then add the numbers.
The two methods are mathematically equivalent. Thus,
Average mass
=
(0.9893 × 12 + 0.0107 × 13.003) u
=
(11.872 + 0.139) u
=
12.011 u
Method 2 is probably mathematically simpler, but Method 1 makes it clear that you are determining an average mass.