Question

Calculate leads approximate atomic mass. Leads isotopes : Pb-204, 1.37%,Pb-206,26.6%,Pb-207,20.82%,and Pb-208, 51.55%

Answer 1

Hello!

To find the average atomic mass of the given isotopes of lead, we need to use the formula: Average Mass = (mass of iso-1)(% of iso-1) + (mass of iso-2)(% of iso-2) + (mass of iso-3)(% of iso-3) + ...

The ellipsis means that you can keep on multiplying the mass of an isotope and the percentage of an isotope together and add them until all of the isotopes of a given element is used.

First off, we cannot multiply by a percentage, so we need to convert it into a decimal. All percentages are out of 100, so we divide the percentage by 100 to get it to a decimal.

1) 1.37 / 100 = 0.0137

2) 26.6 / 100 = 0.266

3) 20.82 / 100 = 0.2082

4) 51.55 / 100 = 0.5155

With all of the percentages converted to decimals, we can use our formula to solve for the average atomic mass.

Remember to round to the number of significant figures! In all of the given numbers, when multiplying, the largest number of significant figures is 3. But, when you add numbers, the significant figure is stops at is at the least number of decimal places (for example: 1 + 0.1 = 1 -- because the least number of decimal places is no decimal places).

Average Mass = (204 amu)(0.0137) + (206)(0.266) + (207)(0.2082) + (208)(0.5155)

= 2.79 + 54.8 + 43.1 + 107

= 207.69 → 207 (the least number of decimal places is no decimal places)  

Therefore, lead's approximate atomic mass is 207 amu.