Science Buddies: "Ask an Expert"

I made up different concentrations of ethanol by measuring volumes of absolute ethanol and distilled water as the beginning to an experiment monitoring their absorption of water during a dehydration process. I measured the specific gravity using a hydrometer and corrected for temperature, then calculated the percentage of ethanol in the mixture knowing the specific gravities of 100% ethanol and 100% H2O.

When I completed the calculations I found the calculated ethanol concentration in mixtures to be lower than the actual concentration made up.

To check, I measured the SG for 0, 10, 20, 30... up to 100% ethanol and calculated the concentrations. Rather than giving a straight line, it produced a smooth curve where 0% and 100% ethanol had correct calculated concentration but again the mixtures had a lower than expected calculated concentration:

0% ethanol (100% H20)= calculated concentration from SG reading= 0% ethanol
10% ethanol = 6% ethanol
20% ethanol = 12% ethanol
30% ethanol = 19% ethanol
40% ethanol = 27% ethanol
50% ethanol = 35% ethanol
60% ethanol = 47% ethanol
70% ethanol = 59% ethanol
80% ethanol = 72% ethanol
90% ethanol = 85% ethanol
100% ethanol = 100% ethanol

This is obviously wrong and as I was very careful measuring volumes and reading the hydrometer it is not simply an error. I dont think the hydrometer is incorrect as it would have resulted in a straight line shifted up of down on the y axis. Some other factor must be in effect.

I have used the trendline and solved for x to correct further mesurements in the experiment but I want to know what factor is causing the SG of the ethanol mixtures to be lower than expected.

Any help is appreciated.

Thanks

Hello markfordyce,

Ethanol-water mixtures do not have simply the sum of the mixed volumes. Did you allow for this? Tables of density vs. concentration are available. For example, http://www.handymath.com/cgi-bin/ethano ... bmit=Entry.

WW