Respuesta :
Answer: The molality of solution is 0.122 m and molar mass of unknown substance is 128.16 g/mol
Explanation:
- To calculate the depression in freezing point, we use the equation:
[tex]\Delta T_f=iK_fm[/tex]
where,
[tex]\Delta T_f[/tex] = depression in freezing point = 2.50°C
i = Vant hoff factor = 1 (For non-electrolytes)
[tex]K_f[/tex] = molal freezing point elevation constant = 20.5°C/m
m = molality of solution = ?
Putting values in above equation, we get:
[tex]2.50^oC=1\times 20.5^oC/m\times m\\\\m=\frac{2.50}{1\times 20.5}=0.122m[/tex]
- To calculate the mass of cyclohexane, we use the equation:
[tex]\text{Density of substance}=\frac{\text{Mass of substance}}{\text{Volume of substance}}[/tex]
Density of cyclohexane = 0.779 g/mL
Volume of cyclohexane = 20.0 mL
Putting values in above equation, we get:
[tex]0.779g/mL=\frac{\text{Mass of cyclohexane}}{20.0mL}\\\\\text{Mass of cyclohexane}=(0.779g/mL\times 20.0mL)=15.58g[/tex]
- To calculate the molar mass of substance for given molality, we use the equation:
[tex]\text{Molality of solution}=\frac{m_{solute}\times 1000}{M_{solute}\times W_{solvent}\text{ (in grams)}}[/tex]
where,
[tex]m_{solute}[/tex] = Given mass of solute = 0.2436 g
[tex]M_{solute}[/tex] = Molar mass of solute = ? g/mol
[tex]W_{solvent}[/tex] = Mass of solvent (cyclohexane) = 15.58 g
Molality of solution = 0.122 m
Putting values in above equation, we get:
[tex]0.122=\frac{0.2436\times 1000}{M_{solute}\times 15.58}\\\\M_{solute}=\frac{0.2436\times 1000}{0.122\times 15.58}=128.16g/mol[/tex]
Hence, the molality of solution is 0.122 m and molar mass of unknown substance is 128.16 g/mol
