Question

O ACIDS AND BASES Preparing a strong base solution with a given pH A chemist must prepare 800.0 mL of sodium hydroxide solution with a pH of 13.40 at 25°C. She will do this in three steps: • Fill a 800.0 mL volumetric flask about halfway with distilled water. • Weigh out a small amount of solid sodium hydroxide and add it to the flask. • Fill the flask to the mark with distilled water. Calculate the mass of sodium hydroxide that the chemist must weigh out in the second step. Round your answer to 2 significant digits. X 5 ?

Answer 1

1st find the concentration of NaOH

use:

pH = -log [H+]

13.4 = -log [H+]

[H+] = 3.981*10^-14 M

use:

[OH-] = Kw/[H+]

Kw is dissociation constant of water whose value is 1.0*10^-14 at 25 oC

[OH-] = (1.0*10^-14)/[H+]

[OH-] = (1.0*10^-14)/(3.981*10^-14)

[OH-] = 0.2512 M

So,

[NaOH] = 0.2512 M

volume , V = 8*10^2 mL

= 0.8 L

use:

number of mol,

n = Molarity * Volume

= 0.2512*0.8

= 0.201 mol

Molar mass of NaOH,

MM = 1*MM(Na) + 1*MM(O) + 1*MM(H)

= 1*22.99 + 1*16.0 + 1*1.008

= 39.998 g/mol

use:

mass of NaOH,

m = number of mol * molar mass

= 0.201 mol * 40 g/mol

= 8.038 g

Answer: 8.0 g