Question

1. Measured boiling point of ethanol/water mixture: 79 degrees Celsius     Known value of boiling point mixture: 156.11 degrees Celsius - Comment on the measured boiling point of your distillate vs. the “known” value you looked up prior to the lab session. Calculate the percent error between these numbers.

Answer 1

The purpose of percentage error calculation is to find out how close is the measured value to the theoretical or known value in an analysis. The ethanol and water forms an azeotropic mixture when the composition of the mixture is such that is has ethanol 95.63% and water 4.37%. The mixture on boiling the vapour has the same proportions of constituents as the unboiled mixture. The azeotrope boils at 78.2 degree Celsius. Here the measured value of the ethanol/water mixture is 79°C and the known value is 156.11°C. The measured value shows that the composition of the ethanol-water mixture might be close to the composition of the azeotrope. But since the known value is 156.11°C it would probably an error in the measurement.

The percent error is calculated as:

% error = [(measured value – known value)/known value] x 100

             = [(79 – 156.11)/156.11] x 100

             = (- 77.11/156.11) x 100

             = 49.39% (only the absolute value is considered and the negative sign is omitted)