# Use of Eötvös rule

wishgames
I am attempting to use Eötvös rule
to find the surface tension of water but am not obtaining normal results.

Surface Tension= 2.7*10^(-7)(374-x)*18^(-2/3)
at T=0 Celsius Surface tension = .0114 mN/m with this equation far from the normal 75.7 mN/m
what am I doing wrong?

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## Answers and Replies

Studiot
Scientists normally measure temperature in degrees Kelvin not degrees Celsius

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Scientists normally measure temperature in degrees Kelvin not degrees Celcius

. . . . . because the Zero, used in all other scales is quite arbitrary. It can be based on the fixed freezing point of water, alcohol or anything else. 0K, otoh, is Absolute.
Everybody, I'm sure, has made the mistake at least once in their lives of using °C when they should have used K and arrived at a crazy answer. You are in good company.

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Since it's Tc-T, it will work in either Celsius or Kelvin. However, you have to make sure that either both are Celsius or both are Kelvin. You can't mix and match.

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I think the error is in deriving V from molar mass and density. You should get 18.10-6 m3/mol. The 10-6 seems to have been lost.

wishgames
I'm doing the calculations in Celsius since the critical temperature is in Celsius. Thank you haruspex. Measuring the V in m3/mol made the result 114 mN/m which is closer to the actual surface tension but still off.

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I'm doing the calculations in Celsius since the critical temperature is in Celsius. Thank you haruspex. Measuring the V in m3/mol made the result 114 mN/m which is closer to the actual surface tension but still off.
Yes, I also get 114 (after correcting the typo in the OP; you had 2.7 instead of 2.1). In the wikipedia entry it mentions subtracting another 6 degrees, but that doesn't help much. Curiously, that leaves the value almost exactly 3/2 times too high.