When calculating pH, it is always necessary to consider what is the source of oxonium cations in a given environment.
Strong monosaturated acids[edit | edit source]
For strong monosaturated acids the dissociation follows the equation
For the calculation we assume:
- the substance quantity of
according to the above equation will be the same as 
, which, given an identical volume, is also true for the concentration, i.e. ![{\displaystyle [\mathrm {H} _{3}\mathrm {O} ^{+}]=[\mathrm {A} ^{-}]}](https://wikimedia.org/api/rest_v1/media/math/render/svg/884e76b2536ce7da3ed060d0cf71599568ce60e8)
;
- all acid - because it is a strong acid - is converted into a , therefore we will mark
![{\displaystyle [\mathrm {A} ^{-}]}](https://wikimedia.org/api/rest_v1/media/math/render/svg/4081d3ad257c85039ca6a3864c7da1d76e5f59f2)
its concentration, i.e. ![{\displaystyle [\mathrm {A} ^{-}]=c_{\mathrm {HA} }}](https://wikimedia.org/api/rest_v1/media/math/render/svg/dbe129c33d984d5d97e1a4d051422e9971b03d59)
Let's deduce:
![{\displaystyle \mathrm {pH} =-\log \ [\mathrm {H} _{3}\mathrm {O} ^{+}]=-\log \ [\mathrm {A} ^{-}]=-\log \ c_{\mathrm {HA} },}](https://wikimedia.org/api/rest_v1/media/math/render/svg/1cb373abb61ba8564af3e7605fffe24a3c687f7b)
and to calculate the pH we get the formula
Strong monosaturated bases[edit | edit source]
For strong monosaturated bases the dissociation follows the equation
We assume, as in the case of strong monosaturates, that:
- the amount, or concentration, of hydroxide ions and the resulting
is the same according to the above chemical equation, i.e. ![{\displaystyle [\mathrm {OH} ^{-}]=[\mathrm {B} ^{+}]}](https://wikimedia.org/api/rest_v1/media/math/render/svg/c39fe60b34a30c8b36a20fa693f8ec176a4b168c)
;
- dissociation occurs completely, i.e.
![{\displaystyle [\mathrm {B} ^{+}]=c_{\mathrm {BOH} }.}](https://wikimedia.org/api/rest_v1/media/math/render/svg/42fd6f37d6fe4284f4047fd44a84d94d4d3c1da7)
The calculation is therefore analogous, we just have to remember that unlike acids, the base is not a source of oxonium cations, but takes oxonium cations from the environment (see the theory of acids and bases), so we add from the equation for the ionic product of water:
![{\displaystyle [\mathrm {H} _{3}\mathrm {O} ^{+}]={\frac {K_{w}}{[\mathrm {OH} ^{-}]}}}](https://wikimedia.org/api/rest_v1/media/math/render/svg/b6d90de41502d92f8d6f48fd5ee9a825644cf47f)
and from these assumptions, we deduce
![{\displaystyle \mathrm {pH} =-\log \ [\mathrm {H} _{3}\mathrm {O} ^{+}]=-\log \ {\frac {K_{w}}{[\mathrm {OH} ^{-}]}}=\log \ [\mathrm {OH} ^{-}]-\log K_{w}=\log \ [\mathrm {B} ^{+}]-\log K_{w}=\log c_{\mathrm {BOH} }-\log K_{w}.}](https://wikimedia.org/api/rest_v1/media/math/render/svg/23b05e7971fa6d480a740b21c4ed021d83272821)
Calculate the pH at 25 °C using the formula
Strong dibasic acids dissociate according to the equation
we assume, then:
- complete dissociation, i.e.
![{\displaystyle c_{\mathrm {H} _{2}\mathrm {A} }=[\mathrm {A} ^{2-}];}](https://wikimedia.org/api/rest_v1/media/math/render/svg/ca307ee2da428c7570828c88ad56dc3376e0add6)
- however, the amount of oxonium cations and the amount of formed
is - in contrast to monosaturated acids - in a ratio of 1:2, i.e. ![{\displaystyle [\mathrm {H} _{3}\mathrm {O} ^{+}]=2\cdot c_{\mathrm {H} _{2}\mathrm {A} }.}](https://wikimedia.org/api/rest_v1/media/math/render/svg/e950708b61264ddb766badfdc6b805850a61e4d6)
From this we derive ![{\displaystyle \mathrm {pH} =-\log \ [\mathrm {H} _{3}\mathrm {O} ^{+}]=-\log \ [\mathrm {A} ^{2-}]=-\log(2\cdot c_{\mathrm {H} _{2}\mathrm {A} })=-\log 2-\log c_{\mathrm {H} _{2}\mathrm {A} }}](https://wikimedia.org/api/rest_v1/media/math/render/svg/09618c3da3652498c06d482a300b3434f1ba7252)
and the pH is calculated according to the formula
Strong dibasic bases dissociate according to the equation
as we assume for monosaturated bases and dibasic acids:
- complete dissociation, i.e.
![{\displaystyle c_{\mathrm {B(OH)} _{2}}=[\mathrm {B} ^{2+}];}](https://wikimedia.org/api/rest_v1/media/math/render/svg/be88aabbe2ccea758989f9b9888a89cd2fd279e9)
- concentration of the formed
and the concentration of hydroxide anions is in the ratio 1:2, i.e.![{\displaystyle [\mathrm {OH} ^{-}]=2\cdot [\mathrm {B} ^{2+}]}](https://wikimedia.org/api/rest_v1/media/math/render/svg/c7086384b32cd0574e14400d68f52ca51aa69eb3)
, in addition, according to the previous assumption ![{\displaystyle [\mathrm {OH} ^{-}]=2\cdot c_{\mathrm {B(OH)} _{2}}}](https://wikimedia.org/api/rest_v1/media/math/render/svg/1bfe4db4440e5eb3dd30535fbf486e61cac372d4)
- hydroxide anions drain oxonium cations from the environment, .
Then we derive
![{\displaystyle -\log[\mathrm {H} _{3}\mathrm {O} ^{+}]=-\log {\frac {K_{w}}{[\mathrm {OH} ^{-}]}}=\log \ [\mathrm {OH} ^{-}]-\log K_{w}=\log(2\cdot c_{\mathrm {B(OH)} _{2}})-\log K_{w}=\log 2+\log \ c_{\mathrm {B(OH)} _{2}}-\log K_{w}}](https://wikimedia.org/api/rest_v1/media/math/render/svg/ffc7bb1b843d4d3c6dda7b269d6594fbb38f48e6)
and the pH at 25 °C is calculated according to the formula
