Question

element r has 3 isotopes: isotope atomic mass (amu) abundance (%) r - 35 34.9689 amu 75.20% r - 37 36.9659 amu 20.30% r - 36 35.9675 amu % fill in the missing abundance and calculate the average atomic mass of element r. the average atomic mass of element r is: amu

Explanation:

Step1: Calculate the missing abundance

The sum of the abundances of all isotopes is 100%. Let the abundance of R - 36 be $x$. Then $x=100-(75.20 + 20.30)=4.50$%

Step2: Calculate the average atomic mass

The formula for average atomic mass $A=\sum_{i = 1}^{n}m_i\times p_i$, where $m_i$ is the atomic mass of isotope $i$ and $p_i$ is its fractional abundance.
$A=(34.9689\times0.7520)+(36.9659\times0.2030)+(35.9675\times0.0450)$
$A = 34.9689\times0.7520=26.2966128$
$36.9659\times0.2030 = 7.4940777$
$35.9675\times0.0450=1.6185375$
$A=26.2966128 + 7.4940777+1.6185375 = 35.409228$ amu

Answer:

The missing abundance is 4.50%, and the average atomic mass of Element R is 35.409228 amu