Third Law of Thermodynamics

The third law of thermodynamics states that at T = 0 K (absolute zero), a perfectly ordered crystalline substances has NO disorder (S = 0).

We can prove this with the formula S=k(ln W).  At absolute zero, there is no motion of the molecules. So, there's only ONE possible arrangement (W = 1).

So 
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For reference, you can calculate the entropy of a system using the formula
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where k is the Boltzmann Constant and W is the number of possible arrangements of a particular state.