A solution has a pOH of 6.39. What is the solutions pH? (You must answer to the correct number of s.d.'s)
For the acid-base equilibrium that operates in water.....
We know that at under standard conditions,
This defines the acid-base equilibrium, the autoprotolysis of water.
We can take
And on rearrangement,
And given that
Now this might seem a lot of work, but only because I derived the equation. You must be able to use the relationship...........
These logarithmic terms were introduced back in the day, before the advent of electronic calculators. Log tables, printed values of
pOH 6.39 gives pH =7.61
For this question, you can use the rule:
Rewritten this gives us:
In water, the following (ionization) reaction occurs:
Therefore the equilibrium can be written like
Since water is the solvent here, we do not take the concentration of water into consideration (is very big). We obtain the following expression:
This means we can say:
To get from the
The same is true for the
Now if we take the Log from both sides of the
A mathematics rule tells us that multiplying inside the logarithm function is the same as adding these logarithms. Therefore we get
And now we can use the definitions of pOH and OH! We get:
Which is the same as