Suppose 30.0 mL of 0.100 M Pb(NO3)2 is added to 60.0 mL of 0.150 M KI. What will be the molar concentrations of all the ions in the mixture after equilibrium has been reached?
2 KI(aq) + Pb(NO3)2(aq) → 2 KNO3(aq) + PbI2(s)
This is a double replacement reaction (K and Pb swap positions).
KNO3 is (aq) because all Nitrates are soluble (see solubility rules).
PbI2 is (s) (see solubility rules).
Moles of KI = Volume x Molarity = 0.06 L x 0.150 = 0.009 moles
Moles of Pb(NO3)2 = Volume x Molarity = 0.03 L x 0.100 = 0.003 moles
Pb(NO3)2 is the limiting reactant.
All Pb ions will be in the form PbI2(s) → [Pb2+] = 0.000 M
Moles of I- left = 0.009 moles – moles used to make PbI2
[I–] = Moles of I- left /Final volume
[I–] = (0.009 – 2 x 0.003) /(0.09) = 0.033 M
[I–] = 0.033 M
K+ stays soluble:
[K+] = 0.009/0.09 = 0.100 M
[K+] = 0.100 M
NO3– stays soluble:
[NO3–] = 2 x 0.003 /0.09 = 0.067 M
[NO3–] = 0.067 M
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