Question

what is the average atomic mass of lithium, if it is 90% li - 7, 5% li - 6, and 5% li - 5?
a (90)(7)+(5)(6)+(5)(5)=685
b (0.9)(7)+(0.5)(6)+(0.5)(5)=11.8
c (0.9)(7)+(0.05)(6)+(0.05)(5)=6.85
d (0.9)(0.05)(0.05)+(7)(5)(5)=175.00225

Explanation:

Step1: Convert percentages to decimals

90% = 0.9, 5% = 0.05, 5% = 0.05

Step2: Calculate average atomic mass

The formula for average atomic mass is the sum of (isotope - mass×isotope - abundance). So, we calculate $(0.9)\times(7)+(0.05)\times(6)+(0.05)\times(5)$.
First, $(0.9)\times(7)=6.3$, $(0.05)\times(6) = 0.3$, $(0.05)\times(5)=0.25$.
Then, $6.3 + 0.3+0.25=6.85$

Answer:

C. (0.9)(7)+(0.05)(6)+(0.05)(5)=6.85