Acid dissociation constant

Question

Can someone explain what the hell is going on here: ​ What is the acid dissociation constant Ka of water? Answer is 1.8e-16 Explanation is Ka = Kw / \[water\], where \[water\]=55M ​ ​

Answer 1

Without additional information, it is hard to answer this question. Ka is an acid dissociation constant meaning it tells you how readily an acid will dissociate once you chuck into some water. With these types of constants, you want to write products/reactants and assess what it means to have a large or small Ka. A large Ka would indicate to you that you have lots of products meaning your acid loved to dissociate.