1 plasmid (6kbp) = 7ag

1plasmid  = 6000bp x 660 = 3960,000 (MW) ≒ 4 x 10^6
1mol = 4 x 10^6g = 6 x 10^23 plasmids
1plasmid = 4 x 10^6 / 6 x 10^23g = 0.7 x 10^-18 = 7x 10^-3 fg = 7 ag

1ng plasmid solution contains 1/7 x 10^9 (= 140,000,000) plasmids.