[Vicstar]

For the autoionisation of water, it looks like

2 H2O (l) H3O+ (aq) + OH– (aq)

I thought that becuase it is going from liquid to ions, wouldnt the Entropy be greater than 0. However, I have been told and I have calculated the opposite, that entropy is less than 0. Please Explain any one.

[Yggdrasil]

In both cases, you have two molecules floating around in solution.  Since the reaction doesn't change the number of molecules floating around in solution and it doesn't significantly change the freedom with which these molecules move around, I don't think you can necessarily say that there should be an increase in entropy.