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Home Q & A Board Science-Related Homework Help Chemistry (Moderator: Laser_3)
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mel2017 mel2017
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7 years ago
Calculate the ph of a 0.3 M solution of NH3
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#1
Valued Member
On Hiatus
7 years ago
to find the pH of a strong base one uses the equation pOH=-log[OH-] ( disassociates completely)
but when finding pH of weak base on needs to first set of the equation for the reaction:

NH3 + HOH Rightwards Arrow (NH4+) + (OH-)
Initial [ ] .1M 0M 0M
Change [ ] -x +x +x
Equilibrium [ ] .1 -x x x
Kb= [NH4+][OH-]/ [NH3]= x^2/ .1-x
Kb= 1.8 x 10^ -5 = x^2/ .1-x

*remember to assume x is negligible because of small Kb*
1.8 x 10^ -5 ~ x^2/ .1 x=.0013
x= [OH-] = .0013x

pOH= -log [OH-]= -log(.0013) = 2.87
14- pOH = pH
pH= 11.13

answer: 11
wrote...
#2
Subject Expert
7 years ago
Hi alyssa_19,

All of the steps that you did were correct. However, you mis-read the concentration of NH3 (it's 0.3 instead of 0.1). Therefore,  x = 0.0023, and that the pH would be  11.36
wrote...
#3
Valued Member
On Hiatus
7 years ago
thank you for correcting my mistake Slight Smile
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