A chemist must prepare 550.0 mL of nitric acid solution with a pH of 2.00 at 25 °C. He will do this in three steps: Fill a 550.0 mL volumetric flask about halfway with distilled water. Measure out a small volume of concentrated (9.0M) stock nitric acid solution and add it to the flask Fill the flask to the mark with distilled water Calculate the volume of concentrated nitric acid that the chemist must measure out in the second step. Round your answer to 2 significant digits 0- mL.

The correct answer is 0.61 ml

Explanation:

Nitric acid is a strong acid. That means that it dissociates completely in water as follows:

HNO₃ → H⁺ + NO₃⁻

As the dissociation is complete, the concentration of H⁺ ions is equal to the initial concentration of the acid (HNO₃). Thus, the pH can be calculated from the initial concentration of the acid:

pH= -log [H⁺] = -log  [acid]

We want a nitric acid solution with a pH of 2.0. so we first calculate the concentration of acid we need:

2.0 = -log [acid]

10⁻²=  [acid]

The chemist has a stock solution with C= 9.0 M and he/she wants a solution with C= 1 x 10⁻² M and V= 550 ml. We use the equation that relates the initial concentration and volume (Ci and Vi, respectively) of a solution with the final concentration and volume (Cf and Vf, respectively):

Ci x Vi = Cf x Vf

⇒ Vi= (Cf x Vf)/Ci = (1 x 10⁻² M x 550 ml)/9.0 M = 0.611 ml

Summarizing, the chemist must measure 0.611 ml of concentrated solution (9.0 M), add it to the flask and fill the flask to the mark until 550 ml in order to obtain a nitric acid solution with a pH of 2.0.

it is the most commonly accepted one

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