You're calculating your concentration wrong. You want it in "M" units - that is molar concentration. Moles/liter.

So for your example above, you have 0.769 mol Ag, 500 ml water which is 0.5 litres, and that gives a conc of 1.531 M (mol/L). Then you take the negative log of that number to get pAg. You should get -0.185 or so depending on what you used for mol wt of AgNO3 (I used 169.83 for my calcs.)

Work through the RIT notes that I linked to above and see how that goes.