in the leaving cert. chemistry paper could i choose to do three experiment questions in section A and then only five questions in section B ?
Yep. Try do all experiments if you can. Sometimes (rarely) questions about experiments can appear in the section B too so definitely focus on them. Organic Chemistry, Fuels and Heats of Reaction are huge. It depends on what your teacher did with you. But if I were you I'd learn Rates of Reaction and Trends in the periodic table (the atom and it's history) pH and water. It may seem like a lot but if you do questions from exam papers you'll see that it's really repetitive