Question

Will a precipitate form when 0.17 L of 2.1 x 10^-3 M Pb(NO3)2 is added to 0.53 L of 8.5 x 10^-3 M NaCl?

Answer 1

In order to determine if a precipitate will form it is necessary to determine if the reaction quotient Q exceeds the Ksp value.
First write a net ionic equation for the reaction:
Pb(NO3)2(aq) + NaCl(aq) → PbCl2(s) + 2NaNO3(aq)

Net ionic equation:
Pb2+(aq) + 2Cl-(aq) →PbCl2(s)
Write this reaction in terms of Ksp:
Ksp = [Pb 2+] * [Cl-]^2

Use the dilution equation M1V1 = M2V2 to determine the initial concentration of each species as mixed - that is before any reaction has taken place: final volume = 700mL
For Pb(NO3)2:
0.0021 * 170 = M2*700
M of Pb(NO3)2 = 0.00051M

For NaCl :
0.0085*530 = M2*700
M of NaCl = 5/700 = 0.006436M

Pb2+ = 0.00051M
Cl-- = 0.006436M

Using these concentration values, calculate Q the reaction quotient:
Q = [Pb2+] * [Cl-] ^2
Q = 0.00051 * (0.006436)^2
Q = 2.11*10^-8

Now compare this to Ksp for PbCl2= 1.6 x 10^-5

Q > Ksp - Therefore a precipitate would form.
Hence ,in our case precipitate will not form