A chemist must prepare 900.0mL of potassium hydroxide solution with a pH of 13.80 at 25°C. He will do this in three steps: Fill a 900.0mL 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.

The chemist needs 39 grams of potassium hydroxide

Explanation:

Step 1: Data given

Volume = 900 mL = 0.9 L

pH = 13.80

Temperature = 25 °C

Step 2: Calculate pOH

pH + pOH = 14

pOH = 14 - 13.80 = 0.20

Step 3: Calculate [OH-]

pOH = 0.20 = - log[OH-]

[OH-] = 10^-0.20

[OH-] = 0.631

Step 4: calculate moles of KOH

KOH is a strong base, this means [OH-] = [KOH]

[KOH] = moles KOH / volume KOH

moles KOH = [KOH] / volume KOH

moles KOH = 0.631 / 0.9 L

moles KOH = 0.701 moles

Step 5: Calculate mass of KOH

mass KOH = Number of moles KOH * Molar mass KOH

mass KOH = 0.701 moles * 56.106 = 39.33 grams ≈ 39 grams

The chemist must weight out 39 grams of potassium hydroxide