Question

A chemistry student wants to prepare a solution that contains 25% ethanol (by volume) and the rest is water. The enthalpy of mixing for ethanol and water is -21.2 kJ/mol. How much heat is evolved when the student makes 500 mL of the solution?

Answer 1

First, we need to determine the volume of ethanol and water in the 500 mL solution.Volume of ethanol = 25% of 500 mL = 0.25 * 500 mL = 125 mLVolume of water = 500 mL - 125 mL = 375 mLNext, we need to convert the volume of ethanol to moles. The molar volume of ethanol is approximately 58.44 g/mol, and the density of ethanol is approximately 0.789 g/mL.Mass of ethanol = volume of ethanol * density of ethanol = 125 mL * 0.789 g/mL = 98.625 gNow, we can convert the mass of ethanol to moles:Moles of ethanol = mass of ethanol / molar mass of ethanol = 98.625 g / 58.44 g/mol = 1.687 molesSince the enthalpy of mixing for ethanol and water is -21.2 kJ/mol, we can now calculate the heat evolved during the mixing process:Heat evolved = moles of ethanol * enthalpy of mixing = 1.687 moles * -21.2 kJ/mol = -35.76 kJTherefore, when the student makes 500 mL of the 25% ethanol solution, approximately 35.76 kJ of heat is evolved.