pH Scale (Leaving Cert Chemistry): Revision Notes

The pH of a solution is a measure of its acidity or basicity, determined by the concentration of hydrogen ions ($H^+$) present.

Example: Find the pH of a 0.2 M Hydrochloric Acid ($HCl$) Solution

Step 1: Write the dissociation reaction

The first thing to do is recognise that $HCl$ is a strong acid, meaning it fully dissociates (breaks apart) in water.

This means that for every mole of $HCl$ added, one mole of hydrogen ions $H^+$ will be released into the solution.

Dissociation reaction:

Step 2: Determine the concentration of $H^+$ ions

Since $HCl$ fully dissociates, the concentration of hydrogen ions $H^+$ is equal to the concentration of the acid.

So, in a 0.2 M $HCl$ solution, the concentration of $H^+$ ions is also 0.2 M.

Step 3: Calculate the pH

The formula for pH is:

Substitute the $[H^+] = 0.2 \, \text{M}$ into the formula:

Step 4: Interpretation of the pH

The calculated pH is approximately 0.70, which is very low.

This makes sense because $HCl$ is a strong acid, and acids with a low pH are highly acidic.

Example: Find the pH of a 0.2 M Sulfuric Acid ($H_2SO_4$) Solution

Step 1: Write the dissociation reaction

Sulfuric acid, $H_2SO_4$, is a diprotic acid, meaning it releases two hydrogen ions $H^+$ for every molecule that dissociates.

The dissociation happens in two stages, but for simplicity, we'll treat the full dissociation:

Dissociation reaction:

Step 2: Determine the concentration of $H^+$ ions

Since $H_2SO_4$ is diprotic, each mole of sulfuric acid produces two moles of hydrogen ions.

Therefore, if the concentration of sulfuric acid is 0.2 M, the concentration of $H^+$ ions will be twice that:

Step 3: Calculate the pH

Substitute the $[H^+] = 0.4 \, \text{M}$ into the pH formula:

Step 4: Interpretation of the pH

The pH of 0.40 indicates a highly acidic solution, which makes sense for sulfuric acid, a strong diprotic acid that significantly increases the concentration of $H^+$ ions.

Example: Find the pH of a Solution Containing 4.9 g of $H_2SO_4$ in 200 cm³ In this example, you are given mass and volume, so you first need to calculate the molarity of the solution before finding the pH.

Step 1: Calculate the moles of $H_2SO_4$

The molar mass of $H_2SO_4$ is:

To find the number of moles of $H_2SO_4$, use the formula:

Step 2: Calculate the molarity of the solution

Molarity is defined as moles per litre of solution.

You have 0.05 moles of $H_2SO_4$ in 200 cm³ of solution.

Convert the volume to litres:

Now calculate the molarity:

Step 3: Determine the concentration of $H^+$ ions

Since sulfuric acid is diprotic, it releases two $H^+$ ions for every molecule that dissociates.

Thus, the concentration of $H^+$ is:

Step 4: Calculate the pH

Substitute the $[H^+] = 0.50 \, \text{M}$ into the pH formula:

Step 5: Interpretation of the pH

The pH of 0.30 indicates a highly acidic solution, which is expected for sulfuric acid, especially at this concentration.

Exam Tip:

• For strong acids and bases, always write the dissociation equation first.
• Use a calculator for logarithmic operations when calculating pH and pOH.