will i still get an A for higher chemistry

It's alright if you kind of messed up in your Chemistry Assignment, don't worry if its going to affect your final grade if you study hard on unit 1 and 2 as they are mostly written knowledge answers in the exam and understand how to do the calculations for unit 3, inshallah you'll get the A, if you work hard now it will pay off.

Agree that it’s not the end of the world ! You need to get stuck into your revision and I’m sure you will do well. Have you looked at Heriot Watts Scholar website ? It’s brilliant and has lots of stuff and webinars you can join specifically for Highers. Good Luck - you’ve got this.

The assignment is scaled from 20 to 30 marks, which means there’s 150 marks total [post scaling]. 70% of 150 is 105 marks you’d have to achieve [post scaling]- but we’ll say 75% for a “comfortable” A depending on grade boundaries, so 113 marks. 12/20 would be scaled to 18/30, subtracted from 113 marks to get 95 marks you’d need in the final exam [out of 120 available], which is circa 79%. If we take the 70% for an A and assume you got 12/20, you’d need 87/120 or 72.5% in the actual exam.
TLDR- You’d need 72.5% for a borderline A, and 79-80% for what I think would be a comfortable A, in the final exam.