i'm thinking buying the mastering chemistry, but not sure if it is worth the money. Any help?

If your willing to put the time and effort into doing it properly and learning from it. Yes very worth it.

in life sci I believe it's also for marks, though optional, it's easy marks
I don;t think they have that option in engineering, so in that case is would JUST be for practice

Its worth about 10%? I wouldn't buy it if u have to buy it separate for 50$. The tests and exam are all multiple choices. You are better off just studying from the textbook with loads of question and solutions and just second hand experiencing mastering chiem with a friend.