Question

A chemist must prepare 800mL of potassium hydroxide solution with a pH of 13.90 at 25C.?

He will do this in three steps.

1. fill a 800mL volumetric flask about halfway with distilled water.

2. weigh out a small amount of solid potassium hydroxide and add it to the flask.

3. 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.

Answer 1

35.6 g

Step-by-step explanation:

Step 1: Calculate the [H⁺]

We use the following expression

pH = -log [H⁺]

[H⁺] = antilog -pH = antilog -13.90 = 1.259 × 10⁻¹⁴ M

Step 2: Calculate the [OH⁻]

We use the ionic product of water (Kw).

Kw = 10⁻¹⁴ = [H⁺] × [OH⁻]

[OH⁻] = 0.7943 M

Step 3: Calculate the moles of OH⁻

`(0.7943mol)/(L) * 0.800 L = 0.635 mol`

Step 4: Calculate the required moles of KOH

KOH is a strong base that dissociates according to the following equation.

KOH → K⁺ + OH⁻

The molar ratio of KOH to OH⁻ is 1:1. Then, the required moles of KOH are 0.635 moles.

Step 5: Calculate the mass of KOH

The molar mass of KOH is 56.11 g/mol.

`0.635 mol * (56.11g)/(mol) =35.6 g`