Question

Brass is a substitutional alloy consisting of a solution of copper and zinc. A particular sample of red brass consisting of 80.0% Cu and 20.0% Znby mass has a density of 8760kg/m3 .

What is the molality of Zn in the solid solution?

What is the molarity of Zn in the solution?

Answer 1

Brass is a substitutional alloy consisting of a solution of copper and zinc. A particular sample of red brass consisting of 82.0% and 18.0% by mass has a density of 8730 .

Part A: What is the molality of in the solid solution?

Part B: What is the molarity of in the solution?

Brass consists of 82.0 % copper and 18.0 % Zinc
The substance in lesser quantity is considered as the solute.

Part A:
For a 100 g sample of brass,
mass of Cu = 82.0 g
mass of Zn = 18.0 g

Molality = mass of solute(g) * 1000 / atomic mass * mass of solvent (g)

            = 18.0 g * 1000 / (65.38 g/mol * 82.0g)

            = 3.36 m

Part B:

For the same 100 g sample, we need to know the volume

Density = 8730 kg /m^3     {Assuming the units, check if correct}

           = 0.873 g/ cm^3

So volume of solution = mass / density

                               = 100 g / 0.873 g/ cm^3

                               = 114.55 cm^3 or 0.115 L

molarity = mass / (molar mass * volume of solution (L))

             = 18.0 g / (65.38 g/mol * 0.115 L)

             = 2.39 M