pH and pOH of Strong Acids and Bases
Introduced| English |
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| pH/piː eɪtʃ/ |
A tidy scale for acidity
- Concentrations of $\text{H}^+$ span many powers of ten.
- A logarithm squeezes that huge range into one neat number.
- Low numbers mean sour and reactive; high numbers mean slippery.
- One scale runs from battery acid to bleach.
The pH scale
- pH pH值 measures acidity: $\text{pH} = -\log[\text{H}^+]$.
- A lower pH is more acidic; a higher pH is more basic.
- Each unit is a factor of ten in $[\text{H}^+]$.
A lower pH means the solution is...
Lower pH means higher $[\text{H}^+]$ -- more acidic.
Going from pH 5 to pH 3 changes $[\text{H}^+]$ by a factor of...
Two pH units is $10^2 = 100$ times.
pH is the ____ logarithm of $[\text{H}^+]$.
$\text{pH} = -\log[\text{H}^+]$.
pOH and the sum
- pOH measures the hydroxide side: $\text{pOH} = -\log[\text{OH}^-]$.
- At 25 degrees, $\text{pH} + \text{pOH} = 14$.
- Know one and you know the other.
A solution has $\text{pH} = 9$ at 25 degrees. Its pOH?
$\text{pOH} = 14 - \text{pH} = 14 - 9 = 5$.
Strong means fully ionized
- A strong acid ionizes completely, so $[\text{H}^+]$ equals its concentration.
- A strong base gives $[\text{OH}^-]$ equal to its concentration.
- Then just take the negative log.
The pH scale
For a strong acid or base, the pH follows straight from the concentration; pH + pOH = 14.
For a strong acid, $[\text{H}^+]$ equals the acid's concentration.
Strong acids ionize completely, so this shortcut holds.
Find the pH of $0.001\ \text{M}$ strong acid.
- $[\text{H}^+] = 0.001 = 10^{-3}$, since it is fully ionized.
- $\text{pH} = -\log(10^{-3}) = 3$.
Find the pH of a $0.01\ \text{M}$ strong acid.
$[\text{H}^+] = 10^{-2}$, so $\text{pH} = -\log(10^{-2}) = 2$.
pH uses a negative log, so a smaller $[\text{H}^+]$ gives a bigger pH. The "$[\text{H}^+]$ equals concentration" shortcut works only for strong acids and bases (fully ionized), not weak ones. And each pH unit is a 10-fold change, not a small one.
pH $= -\log[\text{H}^+]$ measures acidity, with pOH $= -\log[\text{OH}^-]$ and $\text{pH} + \text{pOH} = 14$. For a strong acid or base, $[\text{H}^+]$ or $[\text{OH}^-]$ equals the concentration, so you just take the negative log. Each unit is a factor of ten.