Question

A solution has a ph of 5.50. what is the poh of this solution? Is the solution acidic, or neutral? Calculate the concentration of [H3O+] and [OH-] in this solution?

Answer 1

If `pH=5.50`, `pOH=8.50`...that was easy...and the solution is slightly ACIDIC... So why?

Explanation:

We interrogate the equilibrium...

`2H_2O(l) rightleftharpoonsH_3O^+ +HO^-`

Now careful measurement under standard conditions gives the extent of this equilibrium...

`[H_3O^+][HO^-]=10^-14` at `298*K`...and `100*kPa`..

WE can take `log_10` OF BOTH SIDES to get...

`log_10{[H_3O^+][HO^-]}=underbracelog(10^-14)_(-14)`

And so....

`log_10[H_3O^+]+log_10[HO^-]=-14`...and multiplying both sides by `-1`....

`-log_10[H_3O^+]-log_10[HO^-]=+14`

But by definition...

`underbrace(-log_10[H_3O^+])_"pH"underbrace(-log_10[HO^-])_"pOH"=+14`

And thus our working relationship...

`"pH+pOH=14"`...and you will need to remember how to use this.