Larutan NaOH, KOH, Ca(OH) 2 , dan Ba(OH) 2 , semuanya mempunyai konsentrasi 1 gram/liter dan semuanya terionisasi sempurna. Berat atom H = 1, O = 16, Na = 23, Ca = 40, dan Ba = 137. Larutan manakah yang memiliki pH paling tinggi?

Menentukan pH NaOH   

Mr NaOH = Ar Na + Ar O + Ar H

Mr NaOH = 23 gram/mol + 16 gram/mol + 1 gram/mol

Mr NaOH = 40 gram/mol

mol NaOH =  = = 0,025 mol

M NaOH =  =  = 0,025 M

[OH^-]   = α x M

            = 1 x 0,025 = 2,5 x 10^-2 M

pOH     = - log [OH^-]

            = - log 2,5 x 10^-2

            = 2 – log 2,5

pH       = 14 – pOH

            = 14 – (2 – log 2,5)

            = 12 + log 2,5

 

Menentukan pH KOH

Mr KOH = Ar K + Ar O + Ar H

Mr KOH = 39 gram/mol + 16 gram/mol + 1 gram/mol

Mr KOH = 56 gram/mol

mol KOH =  =  0,018 mol

M KOH = = 0,018 M

[OH^-]   = α x M   

            = 1 x 0,018 = 1,8 x 10^-2 M

pOH     = - log [OH^-]

            = - log 1,8 x 10^-2

            = 2 – log 1,8 

pH       = 14 – pOH

            = 14 – (2 – log 1,8)

            = 12 + log 1,8

Menentukan pH Ca(OH)₂

Mr Ca(OH)₂ = Ar Ca + 2(Ar O) + 2(Ar H)

Mr Ca(OH)₂ = 40 gram/mol + 32 gram/mol + 2 gram/mol

Mr Ca(OH)₂ = 74 gram/mol

n Ca(OH)₂ =  = 0,014 mol

M Ca(OH)₂ =  = 0,014  M

[OH^-]   = α x M

            = 1 x 0,014 x 2 = 2,8 x 10^-2 M

pOH     = - log [OH^-]

            = - log 2,8 x 10^-2

            = 2 – log 2,8

pH       = 14 – pOH

            = 14 – (2 – log 2,8)

            = 12 + log 2,8

 

Menentukan pH Ba(OH)₂

Mr Ba(OH)₂ = Ar Ba + 2(Ar O) + 2(Ar H)

Mr Ba(OH)₂ = 137 gram/mol + 32 gram/mol + 2 gram/mol

Mr Ba(OH)₂ = 171 gram/mol

n Ba(OH)₂ =  = 0,0058 mol

M Ba(OH)₂ = = 0,0058 M

[OH^-]   = α x M

            = 1 x 0,0058 x 2 = 0,0116 = 1,2 x 10^-2 M

pOH     = - log [OH^-]

            = - log 1,2 x 10^-2

            = 2 – log 1,2

pH       = 14 – pOH

            = 14 – (2 – log 1,2)

            = 12 + log 1,2

Sehingga, dapat disimpulkan bahwa :

nilai pH tertinggi adalah 12 + log 2,8  yang dimiliki oleh Ca(OH)₂.