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).

0.80 g

Explanation:

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

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

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:

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

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