Ans. Given-
          Â
Moles of DNA = 19.5 mmol = 1.95 x 10-8 mol
          Â
[Primer] = 75 uM = 7.5 x 10-5 M
Now,
          Â
Required volume = Moles / Molarity
                                              Â
= (1.95 x 10-8 mol) / (7.5 x 10-5
M)Â Â Â Â Â Â Â Â Â Â Â Â
; [1 M = 1 mol/L]
                                              Â
= (1.95 x 10-8 mol) / (7.5 x 10-5 mol /
L)
                                              Â
= 2.600 x 10-4
LÂ Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â
; [1 L = 103 mL]
                                              Â
= 2.600 x 10-1
mLÂ Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â
; [1 mL = 103 uL]
                                              Â
= 260.0 uL
Therefore,
To make 75 uM primer solution, she need to add 260.0 uL water to the
tube.