I want to make a 10% Lead Nitrate Reagent solution. How do I figure out how much lead nitrate to D.I. water do I use? I am making 100ml

If that is 10% by mass, that will be 10g Pb(NO3)2 in 100 g solution. That means 10 g Pb(NO3)2 + 90 g H2O. It may or may not be exactly 100 mL.

If it is 10% w/v, that will be 10 g Pb(NO3)2 in enough H2O to make 100 mL total volume.