The Ph Scale Reference Sheet Page 3
ADVERTISEMENT
1
2
3
4How can we measure the strength of a particular compound in pH scale? What is its lower or upper
limit? The pH scale ranges from 0 to 14. Let’s see why it is up to 14 only and not more than 14.
+
-
When acidic H
O
and basic OH
combine, they form water. If we study the dissociation of water we
3
can solve the mystery of 14.
O ⇌ H
+
-
2H
O
+ OH
2
3
Or
O ⇌ H
+
-
H
O + H
O
+ OH
2
2
3
+
One molecule of water acts as acid and gives off proton (H
) and other molecule acts as base and
accepts that proton. Thus they establish equilibrium. Let’s calculate Equilibrium constant for above
reaction:
+
-
K = [H
O
][OH
] / [H
O]
3
2
Water is present here as a medium itself and its concentration is constant, so we can include it with K.
Now K will become K
which is called as ionic product of water.
w
+
-
K
= [H
O
] [OH
]
w
3
+
-
-7
The concentration of H
O
and OH
found experimentally is 1.0×10
M. Dissociation of H
O
3
2
+
-
+
-
, that’s why we get equal concentration of H
produces same amount of H
O
and OH
O
and OH
.
3
3
Acidic and basic factors are equal in case of water, that’s why it is neutral in nature.
If we take negative log on both sides of the equation, we get:
+
-
-log K
= -log {[H
][OH
]}
w
+
-
-log K
= -log [H
] + (-log [OH
])
w
-7
-7
= -log [1.0×10
] + (-log [1.0×10
])
= 7+7 = 14
pK
= pH + pOH = 14
w
ADVERTISEMENT
0 votes
Related Articles
Related forms
Related Categories
Parent category: Education