AP Chemistry Questions on Average Atomic Mass

📚 Quick Study Guide

• ⚛️ Average atomic mass is the weighted average of the masses of all isotopes of an element.
• 🔢 It's calculated using the formula: Average Atomic Mass = $\sum$ (isotope mass $\times$ fractional abundance).
• ⚖️ Fractional abundance is the percent abundance divided by 100.
• 🧪 Isotope mass is usually given in atomic mass units (amu).
• 💡 Remember to convert percentages to decimals before using them in the formula.
• 📝 The average atomic mass is found on the periodic table.

🧪 Practice Quiz

1. What is the average atomic mass of an element with two isotopes: Isotope 1 has a mass of 10.0 amu and is 20% abundant, and Isotope 2 has a mass of 11.0 amu and is 80% abundant?
   1. 9.2 amu
   2. 10.2 amu
   3. 10.8 amu
   4. 11.2 amu
2. An element has two isotopes: $^{63}X$ with a mass of 62.93 amu and a natural abundance of 69.17%, and $^{65}X$ with a mass of 64.93 amu and a natural abundance of 30.83%. Calculate the average atomic mass of element X.
   1. 63.55 amu
   2. 63.93 amu
   3. 64.55 amu
   4. 64.93 amu
3. Magnesium has three isotopes: $^{24}Mg$ (mass = 23.985 amu, abundance = 78.99%), $^{25}Mg$ (mass = 24.986 amu, abundance = 10.00%), and $^{26}Mg$ (mass = 25.983 amu, abundance = 11.01%). What is the average atomic mass of magnesium?
   1. 24.31 amu
   2. 24.50 amu
   3. 25.00 amu
   4. 25.31 amu
4. A sample of chlorine contains 75.77% chlorine-35 (mass = 34.969 amu) and 24.23% chlorine-37 (mass = 36.966 amu). What is the average atomic mass of chlorine?
   1. 35.00 amu
   2. 35.45 amu
   3. 36.00 amu
   4. 37.00 amu
5. Boron has two naturally occurring isotopes, $^{10}B$ and $^{11}B$, with masses of 10.013 amu and 11.009 amu, respectively. If the average atomic mass of boron is 10.81 amu, what is the percent abundance of $^{10}B$?
   1. 20%
   2. 40%
   3. 60%
   4. 80%
6. Gallium has two naturally occurring isotopes, $^{69}Ga$ and $^{71}Ga$, with masses of 68.9256 amu and 70.9247 amu, respectively. The average atomic mass of gallium is 69.723 amu. What is the percent abundance of $^{71}Ga$?
   1. 39.9%
   2. 60.1%
   3. 50.0%
   4. 70.0%
7. Element X has two isotopes. Isotope 1 has a mass of 14.003 amu and a relative abundance of 99.63%. Isotope 2 has a mass of 15.000 amu and a relative abundance of 0.37%. What is the average atomic mass of element X?
   1. 14.00 amu
   2. 14.01 amu
   3. 14.99 amu
   4. 15.00 amu