Taking into account the definition of pH and pOH, the [OH⁻] is 7.08*10⁻¹⁰ M.
First of all, 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⁺]
Being [H₃O⁺]=1.40×10⁻⁵ M, the pH is calculated as:
pH= - log (1.40×10⁻⁵ M)
Solving:
pH= 4.85
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
Being pH= 4.85, pOH is calculated as:
pOH + 4.85= 14
pOH= 14 - 4.85
pOH= 9.15
Replacing in the definition of pOH the concentration of OH⁻ ions is obtained:
- log [OH⁻]= 9.15
Solving
[OH⁻]= 10⁻⁹ ¹⁵
[OH⁻]= 7.08*10⁻¹⁰ M
In summary, the [OH⁻] is 7.08*10⁻¹⁰ M.
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