Question

boron has two isotopes: boron - 10 and boron - 11. which is more abundant, given that the atomic mass of boron is 10.81?
the element copper has naturally occurring isotopes with mass numbers of 63 and 65. the relative abundance and atomic masses are 69.2% for mass = 63 amu, and 30.8% for mass = 65 amu. calculate the average atomic mass of copper.
show work
a(with correct sig figs)
calculate the atomic mass of bromine. the two isotopes of bromine have atomic masses and relative abundance of 78.92 amu (50.69%) and 80.92 amu (49.31%).
show work.
a(with correct sig figs)

Explanation:

Step1: Determine the more abundant boron isotope

The average atomic mass of boron is 10.81. Boron - 10 has a mass of 10 amu and boron - 11 has a mass of 11 amu. Since 10.81 is closer to 11, boron - 11 is more abundant.

Step2: Calculate the average atomic mass of copper

The formula for average atomic mass is $Average\ atomic\ mass=\sum_{i}(Relative\ abundance_{i}\times Atomic\ mass_{i})$.
For copper, with 69.2% (or 0.692) abundance for mass = 63 amu and 30.8% (or 0.308) abundance for mass = 65 amu.
$Average\ atomic\ mass=(0.692\times63)+(0.308\times65)$
$=43.596 + 20.02$
$=63.616\approx63.6$ amu (to 3 significant figures)

Step3: Calculate the average atomic mass of bromine

Using the formula $Average\ atomic\ mass=\sum_{i}(Relative\ abundance_{i}\times Atomic\ mass_{i})$.
For bromine, with 50.69% (or 0.5069) abundance for mass = 78.92 amu and 49.31% (or 0.4931) abundance for mass = 80.92 amu.
$Average\ atomic\ mass=(0.5069\times78.92)+(0.4931\times80.92)$
$=(0.5069\times78.92)=39.952548$
$(0.4931\times80.92)=40.891652$
$Average\ atomic\ mass=39.952548 + 40.891652$
$=80.8442\approx80.8$ amu (to 3 significant figures)

Answer:

1. Boron - 11 is more abundant.
2. The average atomic mass of copper is 63.6 amu.
3. The average atomic mass of bromine is 80.8 amu.