Molecular Mass and Formula Unit Mass

Molecular Mass

When we studied atomic mass, we learned about the mass of individual atoms. Now, let's expand this idea. We can find the molecular mass by adding the atomic masses of all the atoms in a molecule. This tells us how heavy the molecule is in atomic mass units (u).

What is Molecular Mass?

• Molecular mass = sum of atomic masses of all atoms in a molecule.
• Example: Water (H₂O), Carbon Dioxide (CO₂).

Example

Let's calculate the molecular mass of water (H₂O) and HNO₃.

(a) Molecular Mass of Water (H₂O):

• Atomic mass of Hydrogen (H) = 1 u
• Atomic mass of Oxygen (O) = 16 u

Calculation:

• Water has 2 hydrogen atoms and 1 oxygen atom.
• $\text{Molecular mass of } H_2O = 2 \times 1 + 1 \times 16 = 18 \, u$

(b) Molecular Mass of Nitric Acid (HNO₃):

• Atomic mass of Hydrogen (H) = 1 u
• Atomic mass of Nitrogen (N) = 14 u
• Atomic mass of Oxygen (O) = 16 u

Calculation:

• $\text{Molecular mass of } HNO_3 = 1 + 14 + 3 \times 16$
• $= 1 + 14 + 48 = 63 \, u$

Formula Unit Mass

Now, let’s talk about formula unit mass. This is similar to molecular mass, but we use it for ionic compounds. The formula unit represents the simplest ratio of ions in a compound.

Example

Let's calculate the formula unit mass of calcium chloride (CaCl₂).

Calculation:

• Atomic mass of Calcium (Ca) = 40 u

• Atomic mass of Chlorine (Cl) = 35.5 u

• Calcium chloride has 1 calcium atom and 2 chlorine atoms.

• $\text{Formula unit mass of } CaCl_2 = 40 + 2 \times 35.5$

• $= 40 + 71 = 111 \, u$

Questions and Scenarios

Let’s solve some problems together!

Problem 1

Calculate the molecular masses of the following:

• H₂
• O₂
• Cl₂
• CO₂
• CH₄
• C₂H₆
• C₂H₄
• NH₃
• CH₃OH

Answer 1

• H₂ = 2 × 1 = 2 u
• O₂ = 2 × 16 = 32 u
• Cl₂ = 2 × 35.5 = 71 u
• CO₂ = 12 + 2 × 16 = 44 u
• CH₄ = 12 + 4 × 1 = 16 u
• C₂H₆ = 2 × 12 + 6 × 1 = 30 u
• C₂H₄ = 2 × 12 + 4 × 1 = 28 u
• NH₃ = 14 + 3 × 1 = 17 u
• CH₃OH = 12 + 4 × 1 + 16 = 32 u

Problem 2

Calculate the formula unit masses of the following:

• ZnO
• Na₂O
• K₂CO₃

Given atomic masses:

• Zn = 65 u
• Na = 23 u
• K = 39 u
• C = 12 u
• O = 16 u

Answer 2

• ZnO = 65 + 16 = 81 u
• Na₂O = 2 × 23 + 16 = 62 u
• K₂CO₃ = 2 × 39 + 12 + 3 × 16 = 138 u

Fun Scenario Questions

1. Scenario: You have a bottle of water. What is its molecular mass?

   • Answer: Water (H₂O) has a molecular mass of 18 u.

2. Scenario: You want to bake cookies that use sugar (C₁₂H₂₂O₁₁). What is the molecular mass needed?

   • Answer: C₁₂H₂₂O₁₁ = 12×12 + 22×1 + 11×16 = 342 u.

3. Scenario: If you have a formula unit of NaCl, what is its formula unit mass?

   • Answer: NaCl = 23 + 35.5 = 58.5 u.

4. Scenario: You find an unknown gas that is O₂. How would you find its molecular mass?

   • Answer: O₂ = 2 × 16 = 32 u.

5. Scenario: You are given CH₄ (methane) to analyze. What is its molecular mass?

   • Answer: CH₄ = 12 + 4 × 1 = 16 u.

Conclusion

Understanding molecular mass and formula unit mass helps us know how heavy different substances are. This knowledge is important in chemistry. Keep practicing these calculations! They are essential for future studies in science.