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Question

What is the pH of millimolar solution of ammonium hydroxide which is 20% dissociated?


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Solution

Step 1: Find the value of concentration of the NH4OH solution

We know that NH4OH is present in millimolar concentration.
NH4OH=0.001M(1)
The weak base ammonium hydroxide, so NH4OH will dissociate as OH-ions partially.
The dissociation degree of NH4OH is 20%
degree of dissociation is as follows

=20%=20100=0.2(2)

Step 2: Find the value of OH-
From the value concentration of the above OH-ions can be calculated
Concentration of OH-=Concentration of solution ×degree of dissociation
OH-=0.001×0.2=2×10-4.(3)
We know that water has pOH=7

Hence, the concentration of OH-ions will be
pOH=-logOH-...(4)7=-logOH-OH-=10-7..(5)

To calculate the OH- of total concentration, we should add the equation (3) and (5)
totalOH-=2×10-4+10-7totalOH-=10-4(2+0.001)totalOH-=2.001×10-4

Step 3: Find the values of pOH and pH
Insert OH- in the equation (4)

pOH=-log2.001×10-4pOH=3.6988
The value of pH can be simplified by using the equation

pH=14-pOH..(6)pH=14-3.6988pH=10.301

Therefore, A millimolar solution of ammonium hydroxide that is 20% dissociated has a pH of 10.301.


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