PbBr2
k_sp = 6.3E-6

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chem eq:
PbBr2(s)  <-->  Pb2-(aq) + 2Br-(aq)

equilibrium expr:
k_sp = [Pb][Br]^2

       PbBr2(s)  <-->  Pb2+(aq) + 2Br-(aq)
init:  --              0M         0M
rx:    -x              +x         +2x
final: --              x          2x

x_sp = (x)(2x)^2
x_sp = 4x^3
k_sp = 6.3E-6
6.3E-6 = 4x^3
x = (1.575E-6)^(1/3)
x ~= 1.2E-2

[Pb2+] = x
[Pb2+] = 1.2E-2 M

[Pb2+] = (2*[Br-])^2
1.2E-2 = (2*[Br-])^2
sqrt(1.2E-2) = 2*[Br-]
[Br-] = sqrt(1.2E-2) / 2
[Br-] = 5.4E-2 M