General Chemistry Problem Help!?

[KumQuat]

Use the following balanced chemical equation to do your calculations.

Pb(NO3)2 (aq) + 2Kl (aq) -----> PbI2 (s) + 2KNO3 (aq)

How many mg. of Potassium Iodid are required to remove all the lead from a sample of drinking water containing 56.90 mg of lead (II) nitrate?

Please help and show work/explain! I do not understand one bit!

[Dr.A]

moles Pb(NO3)2 = 0.05690 g / 331.2 g/mol=0.000172

the ratio between Pb(NO3)2 and KI is 1 : 2

moles KI needed = 0.000172 x 2=0.000344

mass KI = 0.000344 mol x 166.0 g/mol=0.05710 g => 57.10 mg