J.R. S. answered 10/17/21
Ph.D. University Professor with 10+ years Tutoring Experience
2NH4I(aq) + Pb(NO3)2aq ==> PbI2(s) + 2NH4NO3(aq) ... balanced equation
moles Pb(NO3)2 present = 981 ml x 1 L / 1000 ml x 0.680 mol/L = 0.6671 moles
moles NH4I required = 0.6671 mols Pb(NO3)2 x 2 mol NH4I / mol Pb(NO3)2 = 1.334 mols NH4I needed
Volume of NH4I needed = 1.334 mols x 1 L / 0.510 mol = 2.62 L NH4I needed
Moles of PbI2 formed = 0.6671 mols Pb(NO3)2 x 1 mol PbI2 / 1 mol Pb(NO3)2 = 0.667 mols PbI2 formed