Sebanyak 25 gram Ca(OH)₂ dilarutkan dalam 5 liter air. Tentukanlah pH larutan itu. (Ar H = 1 ; O = 16 ; Ca = 40)

M Ca(OH)2 = 25 gram
V larutan = 5 L
Ar H = 1 , O = 16 , Ca = 40

Ditanya : pH larutan ?

Jawab :

n Ca(OH)2 :
= m / Mr
= 25 gram / 74 gram/mol
= 0,34 mol

Molaritas Ca(OH)2 :
= n / V
= 0,34 mol / 5 L
= 0,068 mol/L
= 6,8 × 10-² M

[OH-] :
= b. M
= 2 . 6,8 × 10-² M
= 1,36 × 10-¹ M

pOH :
= – log [OH-]
= – log 1,36 × 10-¹
= – ( log 1,36 + log 10-¹)
= – log 1,36 – log 10-¹
= 1 – log 1,36

pH :
= 14 – pOH
= 14 – ( 1 – log 1,36)
= 14 – 1 + log 1,36
= 13 + log 1,36