Concentrations: molality and molar fraction

Tutorial 3

    

Use this schematic "lattice" and →    définitions 1 or →    définitions 2 to resolve the following exercise:

Exercice Calculate the molarity of an ethanol solution of molar fraction $ X_{so} = 0.05$ in water! The density of this solution is about $0.997\frac{g}{mL} $ Solution (Click):

Calculation: $0,05\; mol $ éthanol are contained in a solution of .......... $1\; mol $ in all, so $0.05\; mol $ ethanol and $0.95\; mol $ water The mass of this solution is therefore.......... $m_{sl}=0.05\cdot 45+0.95\cdot18=19.35\; g$ The volume of this solution is therefore .......... $V_{sl}=\frac{m_{sl}}{\rho_{sl}}$ = $\frac{19.35}{0.997}= 19.41\; mL$ The molarity of ethanol is .......... $[so]=\frac{n_{so}}{V_{sl}}$ = $\frac{0.05}{0.01941}= 2.57\; \frac{mol}{L}$