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General ScienceChemistry: Atoms and Molecules

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The formula unit mass of Calcium Chloride ( $CaCl_2$ ) is: (Atomic masses: Ca=40, Cl=35.5)

75.5 u

111 u

110 u

75 u

Correct Answer

🔬 SC • Science Concept & PrincipleGeneral ScienceChemistry: Atoms and Molecules

Option B

111 u

Quick Summary:

The formula unit mass of an ionic compound is calculated by taking the sum of the atomic masses of all atoms present in its formula unit. For $CaCl_2$ , this involves summing one calcium atom and two chlorine atoms.

🔬SCScience SolutionConcept & Principle

💡 Explanation

💡 Everyday Analogy (Real-World Intuition)

Think of it like calculating the total weight of a grocery bag where you have one heavy item weighing 40 grams and two identical lighter items weighing 35.5 grams each.

🧠 Memory Mnemonic / Shortcut Aid

SUM = Sum the Units to get the Mass.

🔢 Key Formulas

$\text{Formula Unit Mass} = \sum (\text{Atomic masses of all atoms in the formula})$

⚙️ Working Principle

The principle relies on Dalton's atomic theory, where the mass of a molecule or formula unit is the arithmetic sum of the constituent atomic masses. We use the formula: $\text{Mass} = \text{Atomic mass of Ca} + 2 \times \text{Atomic mass of Cl}$ . By substituting $40$ for Ca and $35.5$ for Cl, we get $40 + 2(35.5) = 40 + 71 = 111 \text{ u}$ .

📌 Key Points

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Ionic compounds like $CaCl_2$ do not exist as distinct molecules, so we use 'formula unit mass' instead of 'molecular mass'.
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Atomic mass unit (u) is defined as $1/12$ of the mass of a carbon-12 atom.
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Calculations must account for the stoichiometry defined by the subscripts in the chemical formula.

✅ Advantages

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Provides a standardized way to compare the weight of different chemical substances.

❌ Disadvantages / Limitations

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Does not account for the arrangement of ions in the crystal lattice.

🛠️ Applications / Uses

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Essential for calculating the number of moles in a given mass of substance.
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Used in stoichiometry to determine reactants and products in chemical equations.

📄 Additional Information

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Atomic mass of $Ca = 40 \text{ u}$
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Atomic mass of $Cl = 35.5 \text{ u}$
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Option A is incorrect as it represents $40 + 35.5$ . Option C is incorrect as it neglects the decimal in chlorine mass. Option D is incorrect as it miscalculates the sum entirely.

📊 Diagram / Illustration

✅

B is correct — The formula unit mass of $CaCl_2$ is calculated as $40 + 2(35.5) = 111 \text{ u}$ .

Core Concepts Used

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Atomic Mass Formula Unit Mass Ionic Compounds Stoichiometry

💡 EXAM TIP

Always double-check subscripts in the chemical formula before multiplying atomic masses; students often forget the ' $2$ ' in $CaCl_2$ .

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