How many moles of naf must be dissolved in 1.00 liter of a saturated solution of pbf2 at 25˚c to reduce the [pb2+] to 1 x 10–6 molar? (ksp pbf2 at 25˚c = 4.0 x 10–8)

It depends on the subject for me. some exams are easier than others so i usually go through it and answer questions i know based on my general knowledge, then double check after. going through the lesson can a lot  because the exams can be  very lesson-specific, but sometimes they are ridiculously long so a lot of people don't take the time. i did find that, when i went through the lessons, i was making 100's on my exams vs strictly using + the internet and  i make 90's. 

4based on his attributes it can be concluded that through history could have "compelled" him to act in this way.

Ok, you asked the question and posted the answer on there. what do you need?