What volume (in microliters) of 37.5 mM HCl should be used to make 30 mL of a pH 4.0 solution ?

Answer 1

Use 10^-pH to find [H+] from rearrangement of pH = -log [H+]

10^-4 = 1.0E^-4 . This is the concentration of the hydrogen ion needed in the final solution.

Use the fact that you are making 30 mL (0.030 L) and dimensional analysis to convert to moles of hydrogen needed. Volume (in Liters) x Molarity = Moles

Once you have moles of hydrogen, place that in the upper left hand corner of a conversion chart and use dimensional analysis to convert to microliters:

3 E-6 moles x 1000 mMoles x 1 L x 1.0 E6 uL = 80 uL

1 1 mole 37.5 mM 1 L