The chemist the count the number of particles (Atoms, Molecules or Formula Unit) in a given number of moles of a substance by using following relationship.
                              Moles  =  # of Particles / 6.022 × 10²³
Or,
                              # of Particles  =  Moles × 6.022 × 10²³
So, from above relation it is found that 1 mole of any substance contains exactly 6.022 × 10²³ particles. Greater the number of moles greater will be the number of particles.
        
             
        
        
        
The formula of the ppt. formed is PbSo4 , which is inslouble.
        
             
        
        
        
Hello! The answer is D
A good note for these is when there are three elements, one being a singular element and another a compound and there’s a single switch, this could show a single-displacement 
Have a good day gamer.