Question

The molal freezing point constant for copper is 23 °C/m. If pure copper melts at 1083°C, what will be the melting point of a brass made of 13.4 mass percent Zn and the remainder Cu?

Answer 1

Answer: The freezing point of brass is 1028.57°C

Explanation:

We are given:

13.4 mass percent of zinc in brass

This means that 13.4 grams of zinc is present in 100 g of brass

Mass of copper = [100 - 13.4] g = 86.6 g

Depression in freezing point is defined as the difference in the freezing point of pure solution and freezing point of solution.

The equation used to calculate depression in freezing point follows:

$\Delta T_f=\text{Freezing point of pure solution}-\text{Freezing point of solution}$

To calculate the depression in freezing point, we use the equation:

$\Delta T_f=iK_fm$

Or,

$\text{Freezing point of pure solution}-\text{Freezing point of solution}=i\times K_f\times \frac{m_{solute}\times 1000}{M_{solute}\times W_{solvent}\text{ (in grams)}}$

where,

Freezing point of pure copper = 1083°C

i = Vant hoff factor = 1 (For non-electrolytes)

$K_f$ = molal freezing point elevation constant = 23°C/m

$m_{solute}$ = Given mass of solute (zinc) = 13.4 g

$M_{solute}$ = Molar mass of solute (zinc) = 65.38  g/mol

$W_{solvent}$ = Mass of solvent (copper) = 86.6 g

Putting values in above equation, we get:

$1083-\text{Freezing point of solution}=1\times 23^oC/m\times \frac{13.4\times 1000}{65.38g/mol\times 86.6}\\\\\text{Freezing point of solution}=1028.57^oC$

Hence, the freezing point of brass is 1028.57°C