The density of $1 \mathrm{M}$ solution of a compound ' $X$ ' is $1.25 \mathrm{~g} \mathrm{~mL}^{-1}$. The…
The density of $1 \mathrm{M}$ solution of a compound ' $X$ ' is $1.25 \mathrm{~g} \mathrm{~mL}^{-1}$. The correct option for the molality of solution is (Molar mass of compound $X=85 \mathrm{~g}$ )
$1.165 \mathrm{~m}$
$0.858 \mathrm{~m}$
$0.705 \mathrm{~m}$
$1.208 \mathrm{~m}$
Solution
Relation between Molality ( $m$ ), Molarity (M) and density (d) is
$m=\frac{1000 M}{1000 \times d-M \times M_{\text {solute }}}$
Msolute denotes molar mass of solute.
$m=\frac{1000 \times 1}{(1000 \times 1.25)-(1 \times 85)}=\frac{1000}{1250-85}=\frac{1000}{1165}$
$\mathrm{m}=0.858 \mathrm{~m}$