The first thing to do is to calculate the new concentration of the solution after dilution. We do as follows:
M1V1 = M2V2
(3.0 M).006 L = M2 (18 L)
M2 = 0.001 M HNO3
When in solution it dissociated into ions as follows:
HNO3 = H+ + NO3-
[H+] = 0.001 M HNO3 ( 1 mol H+ / 1 mol HNO3 ) = 0.001 M H+
pH = -log[H+] = -log 0.001 = 3
