In certain solvent, phenol dimerizes to the extent of 60%. Its observed molecular mass in that solvent should be

Options

A. `gt 94`
B. `lt 94`
C. `=94`
D. unpredictable

Answer

To solve the problem, we need to determine the observed molecular mass of phenol when it dimerizes to the extent of 60%. Here’s a step-by-step solution:

### Step 1: Understand the Dimerization
Phenol (C6H5OH) dimerizes, meaning two molecules of phenol combine to form a dimer. The extent of dimerization is given as 60%. This means that 60% of the phenol molecules are in the dimer form, while 40% remain as monomers.

### Step 2: Calculate the Molecular Mass of Phenol
The molecular mass of phenol (C6H5OH) can be calculated as follows:
- Carbon (C): 12 g/mol × 6 = 72 g/mol
- Hydrogen (H): 1 g/mol × 6 = 6 g/mol
- Oxygen (O): 16 g/mol × 1 = 16 g/mol

Adding these together gives:
\[ \text{Molecular mass of phenol} = 72 + 6 + 16 = 94 \, \text{g/mol} \]

### Step 3: Determine the Effect of Dimerization on Observed Molecular Mass
When phenol dimerizes, the observed molecular mass will be affected. Since 60% of the phenol is dimerized, we can analyze the composition:
- 60% of phenol molecules are dimers (each dimer has a molecular mass of 188 g/mol, since \( 94 \, \text{g/mol} \times 2 = 188 \, \text{g/mol} \)).
- 40% of phenol molecules remain as monomers (94 g/mol).

### Step 4: Calculate the Average Observed Molecular Mass
To find the average observed molecular mass, we can use the weighted average based on the percentages of monomers and dimers:
\[ \text{Observed Molecular Mass} = (0.4 \times 94) + (0.6 \times 188) \]
Calculating this gives:
\[ \text{Observed Molecular Mass} = (0.4 \times 94) + (0.6 \times 188) = 37.6 + 112.8 = 150.4 \, \text{g/mol} \]

### Step 5: Conclusion
Since the observed molecular mass (150.4 g/mol) is greater than the normal molecular mass of phenol (94 g/mol), we conclude that the observed molecular mass in the solvent is greater than 94.

### Final Answer
The observed molecular mass in that solvent should be **greater than 94**.