Question

The concentration of an organic compound in chloroform is \(6.15\,g\) per \(100\,mL\) of solution. A portion of this solution in a 5 cm polarimeter tube causes an observed rotation of \(-1.2^\circ\). What is the specific rotation of the compound?

• A. \(+12^\circ\)
• B. \(-3.9^\circ\)
• C. \(-39^\circ\)
• D. \(+61.5^\circ\)

Hint:

Always convert tube length from cm to dm in polarimetry. Use \(c\) in \(g/mL\) for correct specific rotation value.

Answer 1

The Correct Option is C

Step 1: Use formula for specific rotation.
\[ [\alpha] = \frac{\alpha_{obs}}{l \times c} \]
Where:
\(\alpha_{obs} = -1.2^\circ\)
\(l = 5cm = 0.5dm\)
\(c =\) concentration in \(g/mL\)
Step 2: Convert concentration.
Given: \(6.15g\) in \(100mL\).
\[ c = \frac{6.15}{100} = 0.0615\,g/mL \]
Step 3: Substitute values.
\[ [\alpha] = \frac{-1.2}{0.5 \times 0.0615} \]
\[ [\alpha] = \frac{-1.2}{0.03075} \approx -39^\circ \]
Final Answer:
\[ \boxed{-39^\circ} \]