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?

Which of the following molecules can rotate the plane of plane-polarized light?

What is a substance called that can rotate the plane of plane-polarized light?

Meso compounds do not show optical activity because

The Fischer projection of $D$-erythrose is shown below.
$D$-Erythrose and its isomers are listed as $P$,$Q$,$R$,and $S$ in Column-$I$. Choose the correct relationship of $P$,$Q$,$R$,and $S$ with $D$-erythrose from Column-$II$.
Column-$I$:
$P$: $CHO-C(OH)(H)-C(OH)(H)-CH_2OH$ (rotated)
$Q$: $CHO-C(H)(OH)-C(OH)(H)-CH_2OH$
$R$: $CHO-C(OH)(H)-C(H)(OH)-CH_2OH$
$S$: $CHO-C(H)(OH)-C(H)(OH)-CH_2OH$ (enantiomer)
Column-$II$:
$1$. Diastereomer
$2$. Identical
$3$. Enantiomer

What is the number of optical isomers for $2, 3-$dibromobutanoic acid?