Taking into account the definition of pH and pOH:
a. it is a basic substance and [OH⁻]= 0.1 M.
b. it is a neutral substance and [H⁺]= 1×10⁻⁷ M.
c. it is a basic substance and [H⁺]= 1×10⁻¹¹ M.
d.it is a acidic substance and [OH⁻]= 2.45×10⁻¹⁰ M.
pH is a measure of acidity or alkalinity that indicates the amount of hydrogen ions present in a solution or substance.
The pH is defined as the negative base 10 logarithm of the activity of hydrogen ions, that is, the concentration of hydrogen ions or H₃O⁺:
pH= - log [H⁺]= - log [H₃O⁺]
The numerical scale that measures the pH of substances includes the numbers from 0 to 14.
The pH value 7 corresponds to neutral substances. Acidic substances are those with a pH lower than 7, while basic substances have a pH higher than 7.
Similarly, pOH is a measure of hydroxyl ions in a solution and is expressed as the logarithm of the concentration of OH⁻ ions, with the sign changed:
pOH= - log [OH⁻]
The following relationship can be established between pH and pOH:
pOH + pH= 14
In this case you know [H⁺]=1.0×10⁻¹³ M
Then, the pH is calculated as:
pH= - log (1.0×10⁻¹³ M)
Solving:
pH= 13
Then, it is a basic substance.
Being pH= 13, pOH is calculated as:
pOH + 13= 14
pOH= 14 - 13
pOH= 1
Replacing in the definition of pOH the concentration of OH⁻ ions is obtained:
- log [OH⁻]= 1
Solving
[OH⁻]= 10⁻¹
[OH⁻]= 0.1 M
Finally, it is a basic substance and [OH⁻]= 0.1 M.
In this case you know [OH⁻]=1.0×10⁻⁷ M
Then, the pOH is calculated as:
pOH= - log (1.0×10⁻⁷ M)
Solving:
pOH= 7
Being pOH= 7, pH is calculated as:
pH + 7= 14
pH= 14 - 7
pH= 7
Then, it is a neutral substance.
Replacing in the definition of pH the concentration of H⁺ ions is obtained:
- log [H⁺]= 7
Solving
[H⁺]= 10⁻⁷
[H⁺]= 1×10⁻⁷ M
Finally, it is a neutral substance and [H⁺]= 1×10⁻⁷ M.
In this case you know [OH⁻]=1.0×10⁻³ M
Then, the pOH is calculated as:
pOH= - log (1.0×10⁻³ M)
Solving:
pOH= 3
Being pOH= 3, pH is calculated as:
pH + 3= 14
pH= 14 - 3
pH= 11
Then, it is a basic substance.
Replacing in the definition of pH the concentration of H⁺ ions is obtained:
- log [H⁺]= 11
Solving
[H⁺]= 10⁻¹¹
[H⁺]= 1×10⁻¹¹ M
Finally, it is a basic substance and [H⁺]= 1×10⁻¹¹ M.
In this case you know [H⁺]=4.0×10⁻⁵ M
Then, the pH is calculated as:
pH= - log (4.0×10⁻¹³ M)
Solving:
pH= 4.39
Then, it is a acidic substance.
Being pH= 4.39, pOH is calculated as:
pOH + 4.39= 14
pOH= 14 - 4.39
pOH= 9.61
Replacing in the definition of pOH the concentration of OH⁻ ions is obtained:
- log [OH⁻]= 9.61
Solving
[OH⁻]= 10⁻⁹ ⁶¹
[OH⁻]= 2.45×10⁻¹⁰ M
Finally, it is a acidic substance and [OH⁻]= 2.45×10⁻¹⁰ M.
Learn more about pH and pOH:
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