Question

A chemist must prepare 900.0 mL of potassium hydroxide solution with a pH of 12.20 at 25°C.

He will do this in three steps:
• Fill a 900.0 mL volumetric flask about halfway with distilled water.
• Weigh out a small amount of solid potassium hydroxide and add it to the flask.
. Fill the flask to the mark with distilled water.
Calculate the mass of potassium hydroxide that the chemist must weigh out in the second step. Round your answer to 2 significant digits and put your answer in grams (g).

Answer 1

0.80 g

Step-by-step explanation:

First we calculate the required pOH from the given pH value:

• pOH = 14 - pH

• pOH = 14 - 12.20 = 1.80

Then we calculate the required concentration of OH⁻, using the pOH:

• pOH = -log[OH⁻]

• [OH⁻] =
  `10^(-1.80)` = 0.0158 M

As the concentration of OH⁻ species is the sames as the concentration of KOH, we need to prepare 900 mL of a 0.0158 M KOH solution:

We calculate how many KOH moles are required, using the concentration and volume:

• Converting 900 mL ⇒ 900 / 1000 = 0.900 L

• moles = 0.0158 M * 0.900 L = 0.01422 mol

Finally we convert 0.01422 moles of KOH to grams, using its molar mass:

• 0.01422 mol * 56 g/mol = 0.80 g