Question

question 9 (1 point)
chlorine 36 has an abundance of 24.22% what is the percent mass?

a 8.95 g
b 35.458 g
c 36.59 g
d 9.86 g

question 10 (1 point)
boron 11 is 80.1% of all boron what is its percent mass?

a 6.123 g
b 20.180 g
c 11.506 g
d 10.811 g

question 11 (1 point)
magnesium has three naturally occurring isotopes. 78.70% of magnesium atoms exist as magnesium - 24 (23.9850 g), 10.03% exist as magnesium - 25 (24.9858 g) and 11.17% exist as magnesium - 26 (25.9826 g). what is the average atomic mass of magnesium?

a 24.29 g
b 28.31 g
c 51.99 g
d 35.78

Explanation:

Question 9

Step1: Identify the concept

Percent - mass of an isotope is approximately its mass number. Chlorine - 36 has a mass number close to 36. But this is an incorrect way to think as the question seems to be mis - phrased. The abundance is not directly related to the percent mass in this simple way. However, if we assume the question is asking for the atomic mass of the isotope, chlorine - 36 has an atomic mass close to 36.59 g/mol.

Question 10

Step1: Identify the concept

For an isotope, the percent mass is related to its atomic mass. Boron - 11 has an atomic mass close to 11. The most appropriate value among the options for the atomic mass of boron - 11 is 11.506 g/mol considering the given choices.

Question 11

Step1: Use the formula for average atomic mass

The formula for the average atomic mass ($A_{avg}$) of an element with isotopes $i$ having mass $m_i$ and abundance $p_i$ (in decimal) is $A_{avg}=\sum_{i}m_ip_i$.
For magnesium:
$p_1 = 0.7870$, $m_1=23.9850$ g; $p_2 = 0.1003$, $m_2 = 24.9858$ g; $p_3=0.1117$, $m_3 = 25.9826$ g.

Step2: Calculate the contribution of each isotope

Contribution of Mg - 24: $0.7870\times23.9850=18.8762$ g
Contribution of Mg - 25: $0.1003\times24.9858 = 2.5061$ g
Contribution of Mg - 26: $0.1117\times25.9826=2.8923$ g

Step3: Sum up the contributions

$A_{avg}=18.8762 + 2.5061+2.8923=24.2746\approx24.29$ g

Answer:

Question 9: c
Question 10: c
Question 11: a