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Q.
Calculate the mass of urea $\left( NH _{2} CONH _{2}\right)$ required in making $2.5 \,kg$ of $0.25$ molal aqueous solution.
Haryana PMTHaryana PMT 2010
Solution:
We know that, Molality $=\frac{\text { moles of solute }}{\text { mass of solvent }(\text { in } kg )}$
$\because$ mass of solvent $=\frac{\text { mass }}{\text { molar mass }}$
So, Molality $=\frac{\frac{\text { mass }}{\text { molar mass }}}{\text { mass of solvent }(\text { in } kg )}$
$0.25=\frac{60}{2.5}$
( $\because$ Molar mass of urea $=60$ )
or Mass $=0.25 \times 2.5 \times 60$
$=37.5 \,g$