Question

Calculate the molecular weight of a substance. In which the solution of this substance in the water has a concentration of 7 percent by weight, has a freezing point

Equal to -0.89 ° C, set Kf value of water = 1.86 ° C / m

Answer 1

Answer : The molecular weight of a substance is 157.3 g/mol

Explanation :

As we are given that 7 % by weight that means 7 grams of solute present in 100 grams of solution.

Mass of solute = 7 g

Mass of solution = 100 g

Mass of solvent = 100 - 7 = 93 g

Formula used :

`\Delta T_f=i* K_f* m\\\\T_f^o-T_f=k_f*\frac{\text{Mass of substance(solute)}* 1000}{\text{Molar mass of substance(solute)}* \text{Mass of water(solvent)}}`

where,

`\Delta T_f` = change in freezing point

`T_f^o` = temperature of pure water =
`0^oC`

`T_f` = temperature of solution =
`-0.89^oC`

`K_f` = freezing point constant of water =
`1.86^oC/m`

m = molality

Now put all the given values in this formula, we get

`(0-(-0.89))^oC=1.86^oC/m* \frac{7g* 1000}{\text{Molar mass of substance(solute)}* 93g}`

`\text{Molar mass of substance(solute)}=157.3g/mol`

Therefore, the molecular weight of a substance is 157.3 g/mol