[EpicWinston]

My professors use www.masteringchemistry.com it is pretty good, but like the other replies indicate- it marks you wrong for the simplest error/notation.

Another one that is good is Mcgraw hill connect which has a learn smart feature. The learn smart feature is amazing, it asks adaptive questions and then pin points what the students needs to study more on.

the last one is https://quest.cns.utexas.edu/ which is the best because it is free!! Although I hated this one because it does not have a hint feature, it was free lol. good luck