we can perform calculations involving ph, poh, and other quantities for pure water when we’re not...
TRANSCRIPT
We can perform calculations involving pH, pOH, and other quantities for pure water when we’re not at 25°C. We’ll do one example here.
Calculations Involving Water at
Temperatures Other Than 25°CExample 1
Before we start the example, it’s very important to remember that when we’re not at 25°C
When we’re NOT at 25°C,
We CANNOT use
When we’re NOT at 25°C,We CANNOT use:
[H3O+][OH–]= 1.00 × 10–14
When we’re NOT at 25°C,We CANNOT use:
[H3O+][OH–]= 1.00 × 10–14
When we’re NOT at 25°C,We CANNOT use:
[H3O+][OH–]= 1.00 × 10–14
And we cannot use pH + pOH = 14.00
When we’re NOT at 25°C,We CANNOT use:
[H3O+][OH–]= 1.00 × 10–14
pH + pOH = 14.00
When we’re NOT at 25°C,We CANNOT use:
[H3O+][OH–]= 1.00 × 10–14
pH + pOH = 14.00
Here’s the example question. We’re given that the value of Kw at 20°C is 6.81 × 10–
15.
The value of Kw at 20°C is 6.81 × 10–15.
a) What is the [H3O+] in water at 20°C?
Question “a” asks us to find the hydronium ion concentration in water at 20°C
The value of Kw at 20°C is 6.81 × 10–15.
b) What is the pH in water at 20°C?
c) What is the [OH–] in water at 20°C?
e) Is water at 20°C acidic, basic, or neutral?
2 153 3
15 83
H O OH H O 6.81 10
H O 6.81 10 8.25 10 M
d) What is the pOH in water at 20°C?
83H O OH 8.25 10 M
a) What is the [H3O+] in water at 20°C?
The Kw, which is hydronium concentration times hydroxide concentration is 6.81 x 10-15
The value of Kw at 20°C is 6.81 × 10–15.
b) What is the pH in water at 20°C?
c) What is the [OH–] in water at 20°C?
e) Is water at 20°C acidic, basic, or neutral?
2 153 3
15 83
H O OH H O 6.81 10
H O 6.81 10 8.25 10 M
d) What is the pOH in water at 20°C?
83H O OH 8.25 10 M
a) What is the [H3O+] in water at 20°C?
Because water is neutral, [OH–]=[H3O+]
The value of Kw at 20°C is 6.81 × 10–15.
b) What is the pH in water at 20°C?
c) What is the [OH–] in water at 20°C?
e) Is water at 20°C acidic, basic, or neutral?
2
3 3
15 83
15H O OH H O
H O
6.
6.81 10 8
81 1
.25
0
10 M
d) What is the pOH in water at 20°C?
83H O OH 8.25 10 M
Because water is neutral
[OH–]=[H3O+]
a) What is the [H3O+] in water at 20°C?
So we can substitute [H3O+] in for [OH–], and get [H3O+]2 = 6.81 × 10–15
The value of Kw at 20°C is 6.81 × 10–15.
b) What is the pH in water at 20°C?
c) What is the [OH–] in water at 20°C?
e) Is water at 20°C acidic, basic, or neutral?
2 153
153
3
8
H O 6.81 10
H O 6.81 10 8.25 10
H OH
M
O
d) What is the pOH in water at 20°C?
83H O OH 8.25 10 M
Substitute [H3O+] for [OH–]
a) What is the [H3O+] in water at 20°C?
So [H3O+] is the square root of 6.81 × 10–15
The value of Kw at 20°C is 6.81 × 10–15.
b) What is the pH in water at 20°C?
c) What is the [OH–] in water at 20°C?
e) Is water at 20°C acidic, basic, or neutral?
15 8
53
3
2 13
H O 6.8
H O OH H O 6.81 1
1 10 8.25 1
0
0 M
d) What is the pOH in water at 20°C?
83H O OH 8.25 10 M
a) What is the [H3O+] in water at 20°C?
Which is 8.25 × 10–8 M
The value of Kw at 20°C is 6.81 × 10–15.
b) What is the pH in water at 20°C?
c) What is the [OH–] in water at 20°C?
e) Is water at 20°C acidic, basic, or neutral?
2 153 3
15 83
H O OH H O 6.81 10
H O 6 8.25.81 10 10 M
d) What is the pOH in water at 20°C?
83H O OH 8.25 10 M
a) What is the [H3O+] in water at 20°C?
The second question asks us the pH in water at 20°C
The value of Kw at 20°C is 6.81 × 10–15.
b) What is the pH in water at 20°C?
c) What is the [OH–] in water at 20°C?
e) Is water at 20°C acidic, basic, or neutral?
2 153 3
15 83
H O OH H O 6.81 10
H O 6.81 10 8.25 10 M
d) What is the pOH in water at 20°C?
83H O OH 8.25 10 M
a) What is the [H3O+] in water at 20°C?
pH is the negative log of the hydronium ion concentration,
The value of Kw at 20°C is 6.81 × 10–15.
b) What is the pH in water at 20°C? c) What is the [OH–] in water at 20°C?
e) Is water at 20°C acidic, basic, or neutral?
2 153 3
15 83
H O OH H O 6.81 10
H O 6.81 10 8.25 10 M
d) What is the pOH in water at 20°C?
83H O OH 8.25 10 M
83pH log log 8.25 10 7.084H O
a) What is the [H3O+] in water at 20°C?
Which is the negative log of 8.25 x 10-8,
The value of Kw at 20°C is 6.81 × 10–15.
b) What is the pH in water at 20°C? c) What is the [OH–] in water at 20°C?
e) Is water at 20°C acidic, basic, or neutral?
2 153 3
15 83
H O OH H O 6.81 10
H O 6 8.25.81 10 M10
d) What is the pOH in water at 20°C?
83H O OH 8.25 10 M
38log 8pH lo .25 10 7.084g H O
a) What is the [H3O+] in water at 20°C?
Which comes out to 7.084
The value of Kw at 20°C is 6.81 × 10–15.
b) What is the pH in water at 20°C? c) What is the [OH–] in water at 20°C?
e) Is water at 20°C acidic, basic, or neutral?
2 153 3
15 83
H O OH H O 6.81 10
H O 6.81 10 8.25 10 M
d) What is the pOH in water at 20°C?
83H O OH 8.25 10 M
83pH log log 8.25 10H O 7.084
a) What is the [H3O+] in water at 20°C?
Question “c” asks for the hydroxide ion concentration in water at 20°C
The value of Kw at 20°C is 6.81 × 10–15.
b) What is the pH in water at 20°C? c) What is the [OH–] in water at 20°C?
e) Is water at 20°C acidic, basic, or neutral?
2 153 3
15 83
H O OH H O 6.81 10
H O 6.81 10 8.25 10 M
d) What is the pOH in water at 20°C?
83H O OH 8.25 10 M
83pH log log 8.25 10 7.084H O
a) What is the [H3O+] in water at 20°C?
We know that the hydronium ion concentration is 8.25 x 10-8 M, and we need to find the hydroxide ion concentration. But be careful, we Cannot use 1.0 x 10-14 divided by hydronium concentration here, because we’re not at 25°C!
The value of Kw at 20°C is 6.81 × 10–15.
b) What is the pH in water at 20°C? c) What is the [OH–] in water at 20°C?
e) Is water at 20°C acidic, basic, or neutral?
2 153 3
15 83
H O OH H O 6.81 10
H O 6 8.25.81 10 10 M
d) What is the pOH in water at 20°C?
83H O OH 8.25 10 M
[H3O+]
83pH log log 8.25 10 7.084H O
a) What is the [H3O+] in water at 20°C?
However, because we’re dealing with pure water, we know that it is neutral.
The value of Kw at 20°C is 6.81 × 10–15.
b) What is the pH in water at 20°C? c) What is the [OH–] in water at 20°C?
e) Is water at 20°C acidic, basic, or neutral?
2 153 3
15 83
H O OH H O 6.81 10
H O 6 8.25.81 10 10 M
d) What is the pOH in water at 20°C?
83H O OH 8.25 10 M
[H3O+]
83pH log log 8.25 10 7.084H O
Neutral
a) What is the [H3O+] in water at 20°C?
Therefore hydroxide ion concentration is equal to hydronium ion concentration.
The value of Kw at 20°C is 6.81 × 10–15.
b) What is the pH in water at 20°C? c) What is the [OH–] in water at 20°C?
e) Is water at 20°C acidic, basic, or neutral?
2 153 3
15 83
H O OH H O 6.81 10
H O 6 8.25.81 10 10 M
d) What is the pOH in water at 20°C?
83OH H O 8.25 10 M
[H3O+]
83pH log log 8.25 10 7.084H O
Neutral
a) What is the [H3O+] in water at 20°C?
Which is 8.25 x 10-8 M
The value of Kw at 20°C is 6.81 × 10–15.
e) Is water at 20°C acidic, basic, or neutral?
2 153 3
15 83
H O OH H O 6.81 10
H O 6 8.25.81 10 10 M
d) What is the pOH in water at 20°C?
83H OOH 8.25 10 M
[H3O+]
c) What is the [OH–] in water at 20°C?
b) What is the pH in water at 20°C?
83pH log log 8.25 10 7.084H O
a) What is the [H3O+] in water at 20°C?
Question “d” asks us to find the pOH in water at 20°C. Remember, we cannot use 14 minus pH to find pOH because we’re not at 25°C.
The value of Kw at 20°C is 6.81 × 10–15.
e) Is water at 20°C acidic, basic, or neutral?
2 153 3
15 83
H O OH H O 6.81 10
H O 6.81 10 8.25 10 M
d) What is the pOH in water at 20°C?
83OH H O 8.25 10 M
c) What is the [OH–] in water at 20°C?
b) What is the pH in water at 20°C?
83pH log log 8.25 10 7.084H O
a) What is the [H3O+] in water at 20°C?
However, because water is neutral, we know that the hydroxide concentration is equal to the hydronium concentration,
The value of Kw at 20°C is 6.81 × 10–15.
e) Is water at 20°C acidic, basic, or neutral?
2 153 3
15 83
H O OH H O 6.81 10
H O 6.81 10 8.25 10 M
d) What is the pOH in water at 20°C?
83OH H O 8.25 10 M
c) What is the [OH–] in water at 20°C?
b) What is the pH in water at 20°C?
83pH log log 8.25 10 7.084H O
Neutral
a) What is the [H3O+] in water at 20°C?
Therefore, the pOH is equal to the pH
The value of Kw at 20°C is 6.81 × 10–15.
e) Is water at 20°C acidic, basic, or neutral?
2 153 3
15 83
H O OH H O 6.81 10
H O 6.81 10 8.25 10 M
d) What is the pOH in water at 20°C?
83OH H O 8.25 10 M
c) What is the [OH–] in water at 20°C?
b) What is the pH in water at 20°C?
83pH log log 8.25 10 7.084H O
pOH = pH
Neutral
a) What is the [H3O+] in water at 20°C?
Which is 7.084.
The value of Kw at 20°C is 6.81 × 10–15.
e) Is water at 20°C acidic, basic, or neutral?
2 153 3
15 83
H O OH H O 6.81 10
H O 6.81 10 8.25 10 M
d) What is the pOH in water at 20°C?
83OH H O 8.25 10 M
c) What is the [OH–] in water at 20°C?
b) What is the pH in water at 20°C?
83pH log log 8.25 10H O 7.084
pOH = pH = 7.084
a) What is the [H3O+] in water at 20°C?
We could also have said that pOH is the negative log of [OH-]
The value of Kw at 20°C is 6.81 × 10–15.
e) Is water at 20°C acidic, basic, or neutral?
2 153 3
15 83
H O OH H O 6.81 10
H O 6.81 10 8.25 10 M
d) What is the pOH in water at 20°C?
83OH H O 8.25 10 M
c) What is the [OH–] in water at 20°C?
b) What is the pH in water at 20°C?
83pH log log 8.25 10 7.084H O
pOH = –log[OH–] =–log(8.25 × 10–8) = 7.084
a) What is the [H3O+] in water at 20°C?
Which is the negative log of 8.25 x 10-8
The value of Kw at 20°C is 6.81 × 10–15.
e) Is water at 20°C acidic, basic, or neutral?
2 153 3
15 83
H O OH H O 6.81 10
H O 6.81 10 8.25 10 M
d) What is the pOH in water at 20°C?
388.OH 2 1H 5O 0 M
c) What is the [OH–] in water at 20°C?
b) What is the pH in water at 20°C?
83pH log log 8.25 10 7.084H O
pOH = –log[OH–] = –log(8.25 × 10–8) = 7.084
a) What is the [H3O+] in water at 20°C?
Which is 7.084
The value of Kw at 20°C is 6.81 × 10–15.
e) Is water at 20°C acidic, basic, or neutral?
2 153 3
15 83
H O OH H O 6.81 10
H O 6.81 10 8.25 10 M
d) What is the pOH in water at 20°C?
83OH H O 8.25 10 M
c) What is the [OH–] in water at 20°C?
b) What is the pH in water at 20°C?
83pH log log 8.25 10 7.084H O
pOH = –log[OH–] = –log(8.25 × 10–8) = 7.084
a) What is the [H3O+] in water at 20°C?
The last question asks if water at 20°C is acidic, basic, or neutral.
The value of Kw at 20°C is 6.81 × 10–15.
e) Is water at 20°C acidic, basic, or neutral?
2 153 3
15 83
H O OH H O 6.81 10
H O 6.81 10 8.25 10 M
d) What is the pOH in water at 20°C?
83OH H O 8.25 10 M
c) What is the [OH–] in water at 20°C?
b) What is the pH in water at 20°C?
83pH log log 8.25 10 7.084H O
pOH = pH = 7.084
a) What is the [H3O+] in water at 20°C?
Here’s where we have to be careful. Notice the pH is above 7. We’re used to thinking that if the pH is above 7, then it is basic. However the 7 is only valid at 25°C.
The value of Kw at 20°C is 6.81 × 10–15.
e) Is water at 20°C acidic, basic, or neutral?
2 153 3
15 83
H O OH H O 6.81 10
H O 6.81 10 8.25 10 M
d) What is the pOH in water at 20°C?
83OH H O 8.25 10 M
c) What is the [OH–] in water at 20°C?
b) What is the pH in water at 20°C?
83pH log log 8.25 10H O 7.084
pOH = pH = 7.084
pH
a) What is the [H3O+] in water at 20°C?
Remember pure water is neutral, no matter what the temperature is.
The value of Kw at 20°C is 6.81 × 10–15.
e) Is water at 20°C acidic, basic, or neutral? Neutral
2 153 3
15 83
H O OH H O 6.81 10
H O 6.81 10 8.25 10 M
d) What is the pOH in water at 20°C?
83OH H O 8.25 10 M
c) What is the [OH–] in water at 20°C?
b) What is the pH in water at 20°C?
83pH log log 8.25 10 7.084H O
pOH = pH = 7.084