A student calculates the formula unit mass of calcium fluoride (CaF₂). If the atomic masses of Ca and F are 40 u and 19 u, respectively, what is the correct formula unit mass?

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Question

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RRB JE CBT 2 (Mechanical Engineering) ReExam Official Paper (Held On: 04 Jun, 2025)

Answer 1

The Correct answer is 78 u.

Key Points

The formula unit mass of Calcium Fluoride (CaF2) is calculated by summing up the atomic masses of its constituent atoms.
The atomic mass of Calcium (Ca) is given as 40 u, and the atomic mass of Fluorine (F) is 19 u.
In the formula for Calcium Fluoride (CaF2), there is 1 atom of Calcium (Ca) and 2 atoms of Fluorine (F).
To calculate the formula unit mass:
- Mass contribution from Calcium = 1 × 40 u = 40 u.
- Mass contribution from Fluorine = 2 × 19 u = 38 u.
- Total formula unit mass = 40 u + 38 u = 78 u.
Thus, the correct formula unit mass of Calcium Fluoride is 78 u.

Additional Information

Formula Unit Mass:
- The formula unit mass refers to the sum of the atomic masses of all atoms in a chemical formula of a compound.
- It is expressed in atomic mass units (u).
Calcium Fluoride (CaF2):
- Calcium Fluoride is an ionic compound consisting of Calcium (Ca²⁺) cations and Fluoride (F^-) anions.
- It is widely used in industries, especially in optics, as it has excellent transparency to ultraviolet and infrared light.