Find the molality and molarity of the sample consisting 75% Copper and 25% Nickel by mass. The computed density of this sample is 0.00639 g/mm3.

Question

.Find the molality and molarity of the sample consisting 75% Copper and 25% Nickel by mass. The computed density of this sample is 0.00639 g/mm3.

Answer 1

The question is a little confusing since they are asking for molality (m) and molarity (M) so there must be a solution of the 2 metals. Yet no volume is given. A density is given, but I don't know if that's for the solution or for the 2 metals combined. If it were to be the density of solution, then that makes no sense since 75% Cu and 25% Ni makes 100% leaving no room for the solvent (H₂O). If it's the density of the metal combination, then we can find the mass of each metal, and the mols of each metal, but we don't know the volume of solution so can't find m and M. Also, I'm wondering if the mm³ is the correct unit of density, or should it be m³? That would make a huge difference. In any event, I will show you the process, even though the units (mm³ or m³) may be wrong and with the above mentioned caveats. I guess we can simply assume a volume of 1 liter and proceed as follows:

1 mm³ = 1x10^-6 L

density = 0.00639 g / mm3 x 1x10 mm3 / L = 6390 g / L = density of the metals

mass of Cu in 1 L = 75% x 6390 g/L = 4793 g

mols Cu = 4793 g x 1 mol / 63.6 g = 75.4 mols Cu

mass of Ni in 1 L = 25% x 6390 g/L = 1598 g

mols Ni = 1598 g x 1 mol / 58.7 g = 27.2 mols Ni

volume of Cu = 0.75 L

volume of Ni = 0.25 L

volume H2O = 1 L = 0.1 L = 0.900 L

mass H2O = 0.900 kg

molality Cu = mols Cu / kg H2O = 75.4 mol / 0.90 kg = 83.8 m

molality Ni = mols Ni / kg H2O =27.2 mol / 0.90 kg = 30.2 m

molarity Cu = mols Cu / L soln = 75.4 mol / 1 L = 75.4 M

molarity Ni = mols Ni / L soln = 27.2 mol / 1 L = 27.2 M