To calculate the average atomic mass, we must take the % abundance of each isotope (in decimal form) and multiply it by its corresponding mass.
(% abundance isotope 1 * mass of isotope 1) + (% abundance isotope 2 * mass of isotope 2) + (% abundance isotope 3 * mass of isotope 3) + .... = average atomic mass.
First, we take each percent abundance and convert it into decimal form.
50.69% ---> 0.5069
49.31% ---> 0.4931
Next, we multiply the decimal forms by the masses.
(0.5069*78.92) + (0.4931*80.92) =
As in everything with chemistry, we must round to the correct number of significant digits/figures. When there are multiple operations, we can underline the significant figure we were supposed to round to at each step. But we wait to round at the end. Keep as many digits as possible.
In this case, when we follow PEMDAS, we have to multiply the parenthesis first. When multiplying, you look at the number of sig figs of each of the given numbers and the answer will contain the fewest number of sig figs. The numbers in the () all contain 4 sig figs. **I have underlined and bolded the significant figure that we need to keep in mind for the final answer**
(0.5069*78.92) + (0.4931*80.92) =
40.004548 + 39.901652
Then, for addition and subtraction, we are supposed to round to the fewest number of decimal places. Even though right now, it looks like there are 6 decimal places, I actually should not look past the underlined digits.
40.004548 + 39.901652 = 79.906232
40.00 + 39.90 ---> round to two decimal places for the final answer
79.906232 ---> 79.91 amu
When looking at the periodic table, only one element comes close to 79.91 amu, which is Bromine.
