Calculate the molality of C2H5OH in a water solution that is prepared by mixing 50.0 mL of C2H5OH with 112.7 mL of H2O at 20°C. The density of the C2H5OH is 0.789 g/mL at 20°C.

Answer 1

molality = moles C₂H₅OH per kg of H₂O

First, find moles of C₂H₅OH:

50.0 ml x 0.789 g / mol x 1 mol C₂H₅OH / 46.1 g = 0.8557 mols C₂H₅OH

Next, convert ml of H₂O to kg of H₂O (assume a density of 1.0 g/ml):

112.7 ml x 1.0 g/ml x 1 kg / 1000 g = 0.1127 kg

Finally, compute the molality:

m = 0.8557 mols / 0.1127 kg

m = 7.59 m (3 sig. figs.)