Two different lab questions: what concentration did I actually make, and how much stock solution do I need to hit a target concentration.
Molarity (M = mol/L): dissolving 11.7g of NaCl (molar mass 58.44 g/mol) in 500mL of solution: moles = 11.7 ÷ 58.44 = 0.200 mol. Molarity = 0.200 mol ÷ 0.500 L = 0.400 M.
Dilution (M₁V₁ = M₂V₂): this is a conservation-of-moles statement - the amount of solute doesn't change when you add solvent, only the concentration and volume do. To make 250mL of 0.10M NaOH from a 2.0M stock: V₁ = M₂V₂ ÷ M₁ = (0.10 × 250) ÷ 2.0 = 12.5mL of stock, then add solvent up to 250mL total (not 250mL of added water - the stock counts toward the final volume).
The dilution formula only works when nothing reacts - it's a statement that the moles of solute stay constant while you add solvent. It does not apply when mixing two different solutions that react with each other (like an acid and a base), since that changes the actual amount of each substance present.