Question

In a sample of 400 lithium atoms, it is found that 30 atoms are lithium-6 (6 amu) and 370 atoms are lithium-
7 (7 amu). Calculate the average atomic mass of lithium.​

Answer 1

Explanation:

To do these problems you need some information: the exact atomic weight for each naturally-occuring stable isotope and its percent abundance. These values can be looked up in a standard reference book such as the "Handbook of Chemistry and Physics." The unit associated with the answer can be either amu or g/mol, depending on the context of the question. If it is not clear from the context that g/mol is the desired answer, go with amu (which means atomic mass unit).

This problem can also be reversed, as in having to calculate the isotopic abundances when given the atomic weight and isotopic weights. Study the tutorial below and then look at the problems done in the reverse direction.

Example #1: Carbon

mass number exact weight percent abundance

12 12.000000 98.90

13 13.003355 1.10

To calculate the average atomic weight, each exact atomic weight is multiplied by its percent abundance (expressed as a decimal). Then, add the results together and round off to an appropriate number of significant figures.

This is the solution for carbon:

(12.000000) (0.9890) + (13.003355) (0.0110) = 12.011 amu

Answer 2

Given: In a sample of 400 lithium atoms, it is found that 30 atoms are lithium-6 (6 amu) and 370 atoms are lithium 7 (7 amu).

To find: We have to find the average atomic mass of lithium.

Solution:

There are 30 lithium-6 (6 amu) atoms are present and 370 lithium-7(7amu) atoms are present.

The total number of lithium atoms is 400.

So, the percentage of lithium 6 will be a ratio of 30 to 400 multiple by 100.

$\frac{30}{400} \times 100 = 7.5\%$

The percentage of lithium 7 will be a ratio of 370 to 400 multiple by 100.

$\frac{370}{400} \times 100 = 92.5\%$

Thus the average atomic mass of lithium will be-

$\frac{(7.5 \times 6 + 92.5 \times 7)}{100} = 6.925$

The average atomic mass of Lithium is 6.925amu.