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I do not understand how to come up and formulate the molecular equation to solve the problem. That is my struggle, not the math.

Lead ions can be precipitated from aqueous solutions by the addition of aqueous iodide: Pb2+(aq)+2I−(aq)→PbI2(s) Lead iodide is virtually insoluble in water so that the reaction appears to go to completion. How many milliliters of 3.550 M HI(aq) must be added to a solution containing 0.700 mol of Pb(NO3)2(aq) to completely precipitate the lead?