Question

The concentration of either the H+ ion or the OH- ion is given for four aqueous solutions at 298 K. For each solution, calculate [H+] or [H-]. State whether the solution is acidic, basic, or neutral.
a.[H+]=1.0 X 10^-13 M c.[OH-]=1.0 X 10^-3 M
b.[OH-]=1.0 X 10^-7 M d.[H+]=4.0 X 10^-5 M

Answer 1

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.

Definition of pH

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.

Definition of pOH

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⁻]

Relationship between pH and pOH

The following relationship can be established between pH and pOH:

pOH + pH= 14

Case a

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.

Case b

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.

Case c

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.

Case d

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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