MYP Chemistry Ph, Titrations and Indicators Notes

The Definition of pH

1. The pH of an aqueous solution is a measure of how acidic or basic it is.
2. It is directly linked to the concentration of hydrogen ions, H⁺(aq), in the solution
3. Mathematically, it is defined as the negative logarithm of the hydrogen ion concentration: $$ \text{pH} = -\log_{10}[\text{H}^+] $$ where $[\text{H}^+]$ is in $\text{mol dm}^{-3}$.
   1. Lower pH → higher $[\text{H}^+]$ → more acidic
   2. Higher pH → lower $[\text{H}^+]$ → more basic
4. This also means that each unit change in pH corresponds to a tenfold change in $[\text{H}^+]$.
   1. pH 3 has 10× more $[\text{H}^+]$ than pH 4.
   2. pH 2 has 100× more $[\text{H}^+]$ than pH 4.

Concentration Solution in Dilute Acids

1. For a strong acid like HCl, we often assume it dissociates completely in water: $$\mathrm{HCl}(\mathrm{aq}) \rightarrow \mathrm{H}^{+}(\mathrm{aq})+\mathrm{Cl}^{-}(\mathrm{aq})$$
2. So: $\left[\mathrm{H}^{+}\right] \approx \text { acid concentration }$
3. When we dilute an acid:
   1. The total number of moles of acid stays the same.
   2. The volume increases, so the concentration decreases.
   3. As $[\text{H}^+]$ decreases, pH increases (solution becomes less acidic).
4. If we dilute by a factor of $F$:
   1. $[\text{H}^+]$ decreases by factor $F$.
   2. pH increases by $\log_{10}{F}$.

Indicators and the pH Scale

How Indicators Work

1. An indicator is a substance that changes colour depending on the pH of the solution.
2. This allows us to see whether a solution is acidic, neutral or basic.
3. In many cases:
   1. The acidic form of the indicator (HIn) has one colour.
   2. The basic form (In⁻) has a different colour.
4. As pH changes, the ratio of HIn to In⁻ changes, and so does the colour.
5. For MYP level, you mainly need to know:
   1. Which colour means acid?
   2. Which colour means base?
   3. In which pH range does each indicator change colour?

Common Laboratory Indicators