Final answer:
The number of atoms in one molecule of MCl₂, where M represents a metal, is 3. This includes one metal atom and two chlorine atoms. The correct answer is option c).
Step-by-step explanation:
The student's question relates to determining the number of atoms in one molecule of MCl₂, with a given formula unit mass of 111 u. The metallic compound MCl₂ suggests that it is made up of one metal atom (M) and two chloride (Cl) ions.
This gives us three atoms in total since MCl₂ is composed of one metal atom and two chlorine atoms bonded together. To calculate the molar mass of MCl₂, we consider the atomic mass of chlorine (Cl), which is approximately 35.45 u. Therefore, two chlorine atoms have a mass of 2 x 35.45 u = 70.9 u.
Subtracting this value from the total formula unit mass of 111 u, we get the mass of the metal (M), which is 111 u - 70.9 u = 40.1 u. The simplest whole number atomic mass near 40.1 u on the periodic table is that of calcium (Ca), which is approximately 40.08 u. Therefore, we can infer that M represents calcium.