Calculate the volume of concentrated hydrobromic acid

Question

A chemist must prepare 750.0 mL

 of hydrobromic acid solution with a pH of 1.50 at 25 C.He will do this in three steps:

1. Fill a 750.0 mL volumetric flask about halfway with distilled water.
2. Measure out a small volume of concentrated (7.0M) stock hydrobromic acid solution and add it to the flask.
3. Fill the flask to the mark with distilled water.

Calculate the volume of concentrated hydrobromic acid that the chemist must measure out in the second step. Round your answer to 2 significant digits.

___ mL

Answer 1

HBr ==> H⁺ + Br^-

For a pH = 1.5, [H⁺] = 1x10^-1.5 = 3.16x10^-2 M

V1M1 = V2M2

V1 = initial volume = ?

M1 = initial molarity = 7.0 M

V2 = 750.0 ml

M2 = 3.16x10^-2

(V1)(7.0) = (750)(3.16x10^-2)

V1 = 3.39 mls

V1 = 3.4 mls (2 sig.figs.)