Acids, Bases and the pH Scale in Daily Life

A brief note on Acids, Bases and the pH Scale in Daily Life. Read it before a test. Try each question at the end before you open its answer.

The pH scale

• Below $7$ acidic, $7$ neutral, above $7$ basic.
• $\text{pH} = -\log_{10}[\text{H}^+]$: for $10^{-n}\ \text{M}$ the pH is $n$. One step is a factor of ten.
• For a strong base, pH $= 14 -$ pOH.

pH in daily life

Neutralisation volumes

Millimoles $=$ M $\times$ mL. For a one to one reaction, $M_1 V_1 = M_2 V_2$; for $\text{H}_2\text{SO}_4 + 2\text{NaOH}$, halve the moles of base.

A worked example

What volume of $0.20\ \text{M}$ sodium hydroxide neutralises $30\ \text{mL}$ of $0.15\ \text{M}$ hydrochloric acid?

1. Acid. $0.15 \times 30 = 4.5\ \text{mmol}$.
2. Base. The same $4.5\ \text{mmol}$ is needed.
3. Volume. $\dfrac{4.5\ \text{mmol}}{0.20\ \text{M}} = 22.5\ \text{mL}$.

Practice with new numbers

A program to try

The program works out three neutralisation volumes, the first being the worked example.

Mistakes to avoid

• Treating pH as an even scale. pH $3$ is not twice as acidic as pH $6$; it is a thousand times.
• Using a strong base as an antacid.
• Rubbing vinegar on a bee sting. It is already acidic.
• Forgetting the ratio for sulphuric acid, which needs two sodium hydroxide.

Check yourself

1. What is the pH of pure water at $25\ ^\circ\text{C}$?

2. Name a mild base used as an antacid.

3. How many times more hydrogen ions does a solution of pH $5$ have than one of pH $6$?

4. What volume of $0.10\ \text{M}$ hydrochloric acid neutralises $14\ \text{mL}$ of $0.25\ \text{M}$ sodium hydroxide?