Question

16) What is the pH of a solution prepared by dissolving 0.45 grams of solid CaO (lime) in enough water 16) to make 2.00 L of aqueous Ca(OH)2 (limewater)?

CaO(s) + H2O(l) Æ Ca2+(aq) + 2 OH-(aq)

A) 11.60

B) 2.40

C) 11.90

D)2.10

Answer 1

we know that

moles = mass / molar mass

given

mass of CaO = 0.45 g

we know that

molar mass of CaO = 56 g

so

moles of CaO = 0.45 / 56

moles of CaO = 8.0357 x 10-3

now consider the given reaction

CaO + h20 ---> Ca+2 + 2OH-

from the above reaction

we get

moles of OH- = 2 x moles of CaO reacted

so

moles of OH- = 2 x 8.0357 x 10-3

moles of OH- = 0.01607

we know that

molarity = moles / volume (L)

given

volume is 2 L

so

[OH-] = 0.01607 / 2

[OH-] = 8.0357 x 10-3

now

we know that

poH = -log [OH-]

so

pOH = -log 8.0357 x 10-3

pOH = 2.095

now

pH = 14 - pOH

so

pH = 14 - 2.095

pH = 11.905

so

the pH is 11.905

so the answer is option C) 11.90