Question

what percent of boron would be boron - 11 *
40%
80%
20%
60%
this is a required question
what is the average mass of boron atom? *
10 amu
11 amu
10.5 amu
10.8 amu
this is a required question

Explanation:

Step1: Recall boron isotopes

Boron has two main isotopes: boron - 10 and boron - 11. The average atomic mass of boron is approximately 10.8 amu. Let the abundance of boron - 11 be $x$ (as a decimal), then the abundance of boron - 10 is $1 - x$. The average atomic mass formula is $A = m_1\times p_1+m_2\times p_2$, where $A$ is the average atomic mass, $m_1$ and $m_2$ are the masses of the isotopes, and $p_1$ and $p_2$ are their abundances. Boron - 10 has a mass of approximately 10 amu and boron - 11 has a mass of approximately 11 amu. So, $10.8=10\times(1 - x)+11\times x$.

Step2: Solve the equation

Expand the right - hand side: $10.8 = 10-10x + 11x$. Combine like terms: $10.8=10 + x$. Subtract 10 from both sides: $x=10.8 - 10=0.8$. Convert $x$ to a percentage: $0.8\times100\% = 80\%$. So the percentage of boron that is boron - 11 is 80%.
For the average mass of boron atom, it is known from the above - mentioned fact and standard atomic mass data that the average atomic mass of boron is 10.8 amu.

Answer:

1. B. 80%
2. D. 10.8 amu