In this lesson, we will learn:
- To apply mole calculations and limiting reagents to find the pH of reacting acid/base solutions.
- To apply expressions for pH and pOH to find the pH of acidic and basic resulting solutions.
- We now know that strong acids produce H3O+ ions in solution that directly lower pH, while strong bases produce OH- in solution that directly raise pH. If a strong acid and base were combined in a solution, the result could be acidic, basic or neutral, depending on how much of the two were used. You can find the resulting pH by calculating the number of moles of substance being added using the equation:
moles (mol) = concentration (mol L-1) volume (L)
This can be done for both OH- and H3O+ ions to find the number of moles. Identify which of the two ionic species is the excess reagent, then subtract the moles of the other species from it. This is done because neutralization in solution will lead to their cancelling each other out in a 1:1 ratio:
H3O+ + OH- → 2H2O
Measuring pH can now be done with the remaining excess ions
- For H3O+ in excess, use the equation:
pH = -log[H3O+]
…To directly find pH.
- For OH- in excess, use the equation:
pOH = -log[OH-]
Now recall that pOH + pH = 14, re-arrange to find:
pH = 14 - pOH
…To find pH.