Calculating Atomic Mass: Examples with Carbon and Oxygen

📚 Quick Study Guide

• ⚛️ Atomic mass is the weighted average of the masses of all isotopes of an element.
• 📝 Isotopes are variants of an element with different numbers of neutrons.
• 🔢 To calculate atomic mass, multiply the mass of each isotope by its natural abundance (as a decimal).
• ➕ Sum the results from each isotope to get the atomic mass.
• 📊 The formula is: Atomic Mass = $(\text{mass of isotope 1} \times \text{abundance of isotope 1}) + (\text{mass of isotope 2} \times \text{abundance of isotope 2}) + ...$

🧪 Practice Quiz

1. What is the first step in calculating the atomic mass of an element?
   1. A) Determine the number of electrons.
   2. B) Identify all isotopes of the element.
   3. C) Find the element on the periodic table.
   4. D) Measure the density of the element.
2. Carbon has two stable isotopes: Carbon-12 (12 amu) with an abundance of 98.9% and Carbon-13 (13 amu) with an abundance of 1.1%. What is the approximate atomic mass of carbon?
   1. A) 12.01 amu
   2. B) 12.50 amu
   3. C) 13.00 amu
   4. D) 12.13 amu
3. Oxygen has three naturally occurring isotopes: Oxygen-16 (15.995 amu, 99.76%), Oxygen-17 (16.999 amu, 0.04%), and Oxygen-18 (17.999 amu, 0.20%). Which of the following contributes the MOST to oxygen's atomic mass?
   1. A) Oxygen-16
   2. B) Oxygen-17
   3. C) Oxygen-18
   4. D) All isotopes contribute equally.
4. If an element has two isotopes, one with a mass of 20 amu and 60% abundance, and the other with a mass of 22 amu, what is its atomic mass?
   1. A) 21.0 amu
   2. B) 21.2 amu
   3. C) 21.4 amu
   4. D) 21.6 amu
5. Which of the following is NOT needed to calculate atomic mass?
   1. A) The mass of each isotope
   2. B) The natural abundance of each isotope
   3. C) The number of protons
   4. D) The number of neutrons
6. An element has two isotopes: Isotope X (mass = 50 amu, abundance = 80%) and Isotope Y (mass = 52 amu, abundance = 20%). What is the atomic mass of the element?
   1. A) 50.4 amu
   2. B) 50.8 amu
   3. C) 51.0 amu
   4. D) 51.2 amu
7. Why is atomic mass a weighted average?
   1. A) Because some isotopes are heavier than others.
   2. B) Because the masses of protons and neutrons are not equal.
   3. C) Because isotopes exist in different proportions in nature.
   4. D) Because electrons have negligible mass.