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AS90310 Describe thermochemical and equilibrium principles
pH Calculations
pH The degree of acidity of a solution is measured using the pH scale.The pH of acids is below 7, neutral substances have a pH of 7, andalkalis have a pH greater than 7.
pH = - log
10
[H
3
O
+
]
or
pH = - log
10
[H
+
]
[
] = concentration in mol L
Example: Calculate the pH of a solution whose hydrogen ionconcentration is 3.65 x 10
mol L
pH = - log
10
3.65 x 10
pH = 4.44 (3 sf)
To calculate log
10
on your calculator use the log key.
[H
3
O
+
] = antilog ( - pH)
or
[H
+
] = antilog ( - pH)
To find the antilog use the 10
x
key (usually the inverse of the log key).
Example: Calculate the [H
3
O
] of a solution whose pH is 8.
[H
3
O
] = antilog ( - 8.45)
[H
3
O
] = 3.55 x 10
mol L
K
W
Water has a pH of 7 which means [H
3
O
] = antilog (- 7) = 10
mol L
.
Since water is neutral, [H
3
O
] = [OH
], so [OH
] = 10
mol L
.
Water ionises only to a very small degree (equilibrium position liesvery much to the left).
2H
2
O
H
3
O
+ OH
or more simply H
2
O
H
+ OH
K
W
= [H
3
O
+
] [OH
-
]
or
K
W
= [H
+
] [OH
-
]
K
W
= [1.00 x 10
-
] [1.00 x 10
-
] = 10
-
This rearranges to
[H
3
O
+
] = K
W
/ [OH
-
]
& to
[OH
-
] = K
W
/ [H
3
O
+
]
We can use K
W
to find the pH of bases if their [OH
] is known
Use 10
= [H
3
O
] [OH
] to calculate [H
3
O
]
Use pH = - log
10
[H
3
O
] to calculate the pH
To find [OH
] when pH is known
Use [H
3
O
] = antilog ( - pH) to calculate [H
3
O
]
Use 10
= [H
3
O
] [OH
] to calculate [OH
]
WANGANUI HIGH SCHOOL
The pH scale
[H
]
1
10
10
10
10
10
10
10
10
10
10
10
10
10
10
[OH
]
10
10
10
10
10
10
10
10
10
10
10
10
10
10
1
pH
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
STRONG ACID
WEAK ACID
*N
WEAK ALKALI
STRONG ALKALI
*N Neutral
Calculators Calculators are different. To obtain antilog you might use shift + log or 2
nd
function +
log. Make sure you know how to do log and antilog on your particular calculator.Also make sure you know how to enter numbers in standard form – and if the answer isgiven in standard form in your calculator display how to write it correctly. Significant figures Rule 1: All non-zero digits are significant.Rule 2: Zeros in front of a number are not significant.Rule 3: Zeros between non-zero digits are significant.Rule 4: Zeros at the end of a number are significant if there is a decimal point in thenumber.
E
Ex
xa
am
mp
pl
le
es
s:
3 h
ha
as
s 3
3 s
si
ig
gn
ni
if
fi
ic
ca
an
nt
t f
fi
ig
gu
ur
re
es
s
2.50 has 3 significant figures3.7 x 10
9
has 2 significant figures
5.05 x 10
4
has 3 significant figures
6 has 1
significant figures but 6.00 has 3 significant figures
There is more than one way to do pHcalculations You must learn
pH = - log
10
[H
+
]
and
[H
+
] = antilog ( - pH)
You will be given
K
W
-
There is another way to do these calculations whichyou may prefer.There is another scale, the
pOH scale
, which is not
very much used.
pOH = - log
10
[OH
-
]
and
[OH
-
]
= antilog ( - pOH)
This isn’t really 2 more equations to bememorized…….You also need to know that
pH + pOH = 14
That’s not really too difficult to remember either.Example:Calculate the [OH
] of a solution of pH 10.
Answer:
pH + pOH = 14 so pOH = 14 – 10.4 = 3.
[OH
] = antilog ( - pOH) so [OH
] = antilog ( - 3.60)
[OH
] = 2.51 x 10
mol L
(3 sf)
WANGANUI HIGH SCHOOL
