Ag+(aq)       +     2 NH3(aq)  Ag(NH3)2+ 
initial             5.0*10-4 M          1.0 M                0
x reacts, 
equil.              5.0*10-4 M - x     1.0 M - x             x

                      [Ag(NH3)2+]                x 
Kc12 =  1.7*107 M-1 = ------------  = ------------------------------
                      [Ag+][NH3]2    (5.0*10-4 M - x) * (1.0 - 2x)2
                                                  obviously x is << 1, 1-2x @ 1.
                           x 
     =  1.7*107 M-1 = --------------
                     5.0*10-4 M - x
       8.5*103 = (1+1.7*107)x

At this point we look a the left-hand side and realize that 1 << 1.7*107. This whole thing reduces quickly to x = 5.0*10-4 M. This implies 100% reaction. and to 2 sig. fig. is the best we can do.

 

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