Actually, you have it a little backward. The capital M is molarity, mols per liter. So, the lead nitrate solution is 0.25 mols per liter. If you have 0.00035 liters, how many mols do you have? You get the answer by multiplying the volume by the molarity, giving you the number of mols. The idea behind how many mols of Pb+2 you have is that lead nitrate is soluble at room temperature and thus the cation and anion dissociate completely in water under normal conditions. This means that for ever mol of Pb(NO3)2 that you have, you get 1 mol of Pb+2 and 2 mols of NO3- dissolved in water.
2. This question doesn't make sense. A solid can't precipitate from a solid; there may be a solid precipitating from solution, but not solid from solid. That's just a solid-solid reaction then. Check the problem.
3. First, write the equation for the dissociation of HCN:
HCN --> H+ + CN-
It's already balanced, so there's no problem there. Now, you find the hydrogen ion concentration just like you said, by using the antilog. Since you know the hydrogen ion concentration, you can use stoichiometry to know what the cyanide ion concentration is. For every H+ you have, you get one CN- according to the chemical equation. Therefore, [ H+ ] = [ CN- ].