Calculate Formula Unit Mass for Compounds

To find the compound with a formula unit mass of 100 u, we need to calculate the mass for each option using the atomic masses of the constituent elements.

Atomic Masses Reference

Approximate atomic masses are:

Step-by-Step Mass Calculations

1. Mass of MgO

Formula Unit Mass (MgO) = Atomic Mass (Mg) + Atomic Mass (O)

$Mass_{MgO} = 24.3 \text{ u} + 16.0 \text{ u} = 40.3 \text{ u}$

2. Mass of $CaCl_2$

Formula Unit Mass ($CaCl_2$) = Atomic Mass (Ca) + 2 * Atomic Mass (Cl)

$Mass_{CaCl_2} = 40.1 \text{ u} + 2 \times 35.5 \text{ u} = 40.1 \text{ u} + 71.0 \text{ u} = 111.1 \text{ u}$

3. Mass of $Na_2CO_3$

Formula Unit Mass ($Na_2CO_3$) = 2 * Atomic Mass (Na) + Atomic Mass (C) + 3 * Atomic Mass (O)

$Mass_{Na_2CO_3} = (2 \times 23.0 \text{ u}) + 12.0 \text{ u} + (3 \times 16.0 \text{ u})$

$Mass_{Na_2CO_3} = 46.0 \text{ u} + 12.0 \text{ u} + 48.0 \text{ u} = 106.0 \text{ u}$

4. Mass of $CaCO_3$

Formula Unit Mass ($CaCO_3$) = Atomic Mass (Ca) + Atomic Mass (C) + 3 * Atomic Mass (O)

$Mass_{CaCO_3} = 40.1 \text{ u} + 12.0 \text{ u} + (3 \times 16.0 \text{ u})$

$Mass_{CaCO_3} = 40.1 \text{ u} + 12.0 \text{ u} + 48.0 \text{ u} = 100.1 \text{ u}$

Conclusion

Comparing the calculated masses:

• MgO: 40.3 u
• $CaCl_2$: 111.1 u
• $Na_2CO_3$: 106.0 u
• $CaCO_3$: 100.1 u

The compound with a formula unit mass closest to 100 u is $CaCO_3$. Therefore, $CaCO_3$ is the correct answer.