An ionic compound of element M and chlorine has the formula, `MCl_3`. The molecular mass of `MCl_3` is 118.5. What will be the molecular mass of the oxide of element M ?

Options

A. 28
B. 44
C. 72
D. 99

Explanation

To find the molecular mass of the oxide of element M, we can follow these steps:

### Step 1: Determine the atomic mass of element M
Given that the molecular mass of the ionic compound \( MCl_3 \) is 118.5 g/mol, we can express this as:
\[ \text{Molecular mass of } MCl_3 = \text{Atomic mass of M} + 3 \times \text{Atomic mass of Cl} \]
The atomic mass of chlorine (Cl) is approximately 35.5 g/mol. Therefore, we can substitute this value into the equation:
\[ 118.5 = \text{Atomic mass of M} + 3 \times 35.5 \]

### Step 2: Calculate the contribution of chlorine
Now, calculate the contribution of chlorine to the molecular mass:
\[ 3 \times 35.5 = 106.5 \]
So, we can rewrite the equation:
\[ 118.5 = \text{Atomic mass of M} + 106.5 \]

### Step 3: Solve for the atomic mass of M
Now, isolate the atomic mass of M:
\[ \text{Atomic mass of M} = 118.5 - 106.5 = 12 \]

### Step 4: Determine the molecular mass of the oxide of M
The oxide of element M can be represented as \( M_2O_3 \). The molecular mass of \( M_2O_3 \) can be calculated as follows:
\[ \text{Molecular mass of } M_2O_3 = 2 \times \text{Atomic mass of M} + 3 \times \text{Atomic mass of O} \]
The atomic mass of oxygen (O) is approximately 16 g/mol. Therefore:
\[ \text{Molecular mass of } M_2O_3 = 2 \times 12 + 3 \times 16 \]

### Step 5: Calculate the molecular mass of the oxide
Now calculate:
\[ 2 \times 12 = 24 \]
\[ 3 \times 16 = 48 \]
Adding these together gives:
\[ \text{Molecular mass of } M_2O_3 = 24 + 48 = 72 \]

### Final Answer
The molecular mass of the oxide of element M is **72 g/mol**.

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