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Q.
If a solution of glucose in water is labelled as $10\% \,w/w$, what would be the molality of the solution if density of solution is $1.2\, g\, mL^{-1}$?
Solutions
Solution:
Let mass of solution $= 100\, g$,
$\therefore $ Mass of glucose $= 10\, g$,
Mass of water $= 90\, g$
No. of moles of $10\, g$ glucose$ = \frac{10}{108} = 0.0555$ mol,
No. of moles of $H_{2}O = \frac{90}{18} = 5$ mol
Volume of solution $= \frac{100}{1.2 \, mL} = 83.33\, mL = 0.0833 \,L$
Molality $ = \frac{\text{number of moles of solute}}{\text{Mass of solvent in kg}}$
$ = \frac{0.0555 \, mol}{0.090 \, kg}$