The formula unit mass of $ ext{CaCl}_2 $ is:

Question

The formula unit mass of $	ext{CaCl}_2 $ is:

Examoogle · Accepted Answer

The correct answer is Option C: 111 u. The formula unit mass of a substance is the sum of the atomic masses of all atoms present in its formula unit. For calcium chloride ($CaCl_2$), it consists of 1 calcium atom ($Ca$) and 2 chlorine atoms ($Cl$).

Step-by-Step Solution & Concepts:
The formula unit mass of a substance is the sum of the atomic masses of all atoms present in its formula unit. For calcium chloride ($CaCl_2$), it consists of 1 calcium atom ($Ca$) and 2 chlorine atoms ($Cl$).

Working Principle: The atomic mass of calcium ($Ca$) is approximately $40	ext{ u}$ and the atomic mass of chlorine ($Cl$) is approximately $35.5	ext{ u}$. Calculating the total formula unit mass: $1 	imes 	ext{Mass of Ca} + 2 	imes 	ext{Mass of Cl} = 1 	imes 40	ext{ u} + 2 	imes 35.5	ext{ u} = 40 + 71 = 111	ext{ u}$.

Key Formulas: $	ext{Formula Mass} = (1 	imes 	ext{At. Mass of Ca}) + (2 	imes 	ext{At. Mass of Cl})$; $	ext{Total Mass} = 40 + 2(35.5) = 111	ext{ u}$

Key Points: Calcium chloride is an ionic compound composed of calcium ions ($Ca^{2+}$) and chloride ions ($Cl^{-}$).; The term 'formula unit mass' is used for ionic compounds instead of 'molecular mass' because they exist as continuous ionic lattices rather than discrete molecules.; Atomic mass of Calcium ($Ca$) = $40	ext{ u}$.; Atomic mass of Chlorine ($Cl$) = $35.5	ext{ u}$.

Applications: Used for road de-icing and dust control.; Used as a desiccant (drying agent) in laboratories and industrial processes.

Additional Info: Option A ($115	ext{ u}$) and Option B ($114	ext{ u}$) are incorrect calculations.. Option D ($110	ext{ u}$) is slightly underestimated.

Answer: C is correct — The formula unit mass of $CaCl_2$ is calculated as $40 + (2 	imes 35.5) = 111	ext{ u}$.

Exam Tip: For ionic compounds like $NaCl$ or $CaCl_2$, always use the term 'formula unit mass' instead of 'molecular mass' in theoretical chemistry questions to avoid negative marking in competitive exams.

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