Solution composition...?

Question

A solution at 25 C is prepared by mixing 100 ml of liquid ethanol C2H5OH (density = 0.789 g/mL), with 200 mL of water (density = 0.997 g/mL). Assuming that the voumes add upon mixing, what are the molarity and molality of the solution, and the mass percent and mole fraction of the ethanol?

Step by step on how to do this problem would be greatly appreciated. thanks.

Answer 1

First, you have to calculate the mass of each component by multiplying their volumes by their densities. You should now have weights for the ethanol and for the water.
Next, calculate the number of moles of each solvent by dividing those weights by their atomic weights.

Now you can begin answering the questions. Molarity is defined as the number of moles of a substance per liter of solution. The total volume of the solution, due to the assumption that the volumes are additive, is 100 ml + 200 ml, giving you 300 ml or 0.3 liters. So, to get the molarity with respect to ethanol, divide the number of moles of ethanol by 0.3 L.

Molality is the number of moles per kilogram of solvent. Do exactly the same as above, except this time, you have to calculate the total Kg of solvent instead of the total volume of solvent. You have to add the mass of ethanol and the mass of water you have calculated from the volumes and the densities. That's your total mass but don't forget to divide by 1000 to get Kg. Then you calculate the molality of ethanol by dividing the number of moles of ethanol by the weight of your total solution in Kg.

The mass percent of ethanol would be the mass of ethanol divided by the total mass of the solution multiplied by 100%. This is just like any other percentage calculation.

The mole fraction of ethanol can be calculated by dividing the number of moles of ethanol by the sum of the moles of ethanol and water.

Good luck.

Answer 2

1. Convert the ethanol to its mass (density x volume)
2. Convert that mass to moles (divide by the Mr, ie 46)
3. Molarity = moles/ vol of solution (0.300 litres).
4. Molality = moles per kilogram of solvent, so convert the 200ml of water to kg (density x volume) and scale up.
5. You have the two masses now, so mass percent is easy.
6. Work out the moles of water by dividing the mass of water by its Mr (18).
7. You now have the two moles, so work out the mole fraction (= moles of ethanol/ total no. of moles).