pH Scale

Definition of pH

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

It is calculated using the following formula:

pH = -\log [H^+]

Where:

$[H^+]$ represents the concentration of hydrogen ions in moles per litre (mol/L).

Features of the pH Scale

The pH scale ranges from 0 to 14, with 7 being neutral.

pH < 7: Acidic solutions (e.g., $HCl$ , hydrochloric acid).
pH = 7: Neutral solutions (e.g., pure water).
pH > 7: Basic (alkaline) solutions (e.g., $NaOH$ , sodium hydroxide). Strong acids like hydrochloric acid ( $HCl$ ) fully dissociate in water, contributing more $H^+$ ions and resulting in a lower pH.

Strong bases like sodium hydroxide ( $NaOH$ ) dissociate fully to produce hydroxide ions ( $OH^-$ ), reducing $H^+$ and resulting in a higher pH.

Measurement of pH

pH can be measured using:
- pH meter: Provides an accurate, direct reading of pH.
- Universal indicator: A solution or paper that changes colour based on the pH of the solution. The colour can be matched to a chart that indicates the corresponding pH value.

Limitations of the pH Scale

The pH scale is only applicable to dilute aqueous solutions.
It provides values between 0 and 14. However, extremely concentrated acids or bases may have pH values outside this range.

Calculations Involving pH

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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 recognize 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:

HCl \rightarrow H^+ + Cl^-

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:

pH = -\log [H^+]

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

pH = -\log (0.2) \approx :highlight[0.70]

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.

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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:

H_2SO_4 \rightarrow 2H^+ + SO_4^{2-}

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:

[H^+] = 2 \times 0.2 \, \text{M} = :highlight[0.4 \, \text{M}]

Step 3: Calculate the pH

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

pH = -\log [H^+]

= -\log (0.4) \approx :highlight[0.40]

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.

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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:

H_2SO_4 \, \text{molar mass} = 2(1) + 32 + 4(16)

= :highlight[98 \, \text{g/mol}]

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

\text{Moles of } H_2SO_4 = \frac{\text{mass}}{\text{molar mass}}

= \frac{4.9 \, \text{g}}{98 \, \text{g/mol}} = :highlight[0.05 \, \text{mol}]

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:

200 \, \text{cm}^3 = :highlight[0.200 \, \text{L}]

Now calculate the molarity:

\text{Molarity} = \frac{0.05 \, \text{mol}}{0.200 \, \text{L}}

= :highlight[0.25 \, \text{M}]

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:

[H^+] = 2 \times 0.25 \, \text{M}

= :highlight[0.50 \, \text{M}]

Step 4: Calculate the pH

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

pH = -\log [H^+]

= -\log (0.50) \approx :highlight[0.30]

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.

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Exam Tip:

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

pH Scale Simplified Revision Notes for Leaving Cert Chemistry

pH Scale

Definition of pH

Features of the pH Scale

Measurement of pH

Limitations of the pH Scale

Calculations Involving pH

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

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

500K+ Students Use These Powerful Tools to Master pH Scale For their Leaving Cert Exams.

Other Revision Notes related to pH Scale you should explore

pH Scale Simplified Revision Notes for Leaving Cert Chemistry

pH Scale

Definition of pH

Features of the pH Scale

Measurement of pH

Limitations of the pH Scale

Calculations Involving pH

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

Example: Find the pH of a 0.2 M Sulfuric Acid (H2SO4H_2SO_4H2​SO4​) Solution

500K+ Students Use These Powerful Tools to Master pH Scale For their Leaving Cert Exams.

Other Revision Notes related to pH Scale you should explore

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

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