Question

A chemist must prepare of 800.0 ml potassium hydroxide solution with a pH of 13.00 at 25°.

She will do this in three steps:
Fill a 800 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 significant digits.

Answer 1

4.48 grams is the mass of potassium hydroxide that the chemist must weigh out in the second step.

Step-by-step explanation:

The pH of the solution = 13.00

pH + pOH = 14

pOH = 14 - pH = 14 - 13.00 = 1.00

`pOH=-\log[OH^-]`

`1.00=-\log[OH^-]`

`[OH^-]=10^(-1.00) M=0.100 M`

`KOH(aq)\rightarrow K^+(aq)+OH^-(aq)`

`[KOH]=[OH^-]=[K^+]=0.100 M`

Molariy of the KOH = 0.100 M

Volume of the KOH solution = 800 mL= 0.800 L

1 mL = 0.001 L

Moles of KOH = n

`Molarity=(Moles)/(Volume(L))`

`0.100 M=(n)/(0.800 L)`

n = 0.0800 mol

Mass of 0.0800 moles of KOH :

0.0800 mol × 56 g/mol = 4.48 g

4.48 grams is the mass of potassium hydroxide that the chemist must weigh out in the second step.