The solutions at the two Pb electrodes of a concentration cell were prepared as follows: Cell A: A mixture of 1.00 mL of 0.0500 M Pb(NO3)2 with 4.00 mL of 0.0500 M KX (the soluble potassium salt of an unspecified monovalent ion X-). Some PbX2(s) precipitates. Cell B: 5.00 mL of 0.0500 M Pb(NO3)2. The cell potential was measured to be 0.06800 V at 25 °C. By use of the Nernst equation, determine the concentration (M) of Pb2+ in the solution of Cell A.