Edexcel International A Level (IAL) Chemistry (YCH11) - Unit 4 - 14.3–14.5 pH and [H⁺] relationships-Study Notes - New Syllabus
Edexcel International A Level (IAL) Chemistry (YCH11) -Unit 4 – 14.3–14.5 pH and [H⁺] relationships- Study Notes- New syllabus
Edexcel International A Level (IAL) Chemistry (YCH11) -Unit 4 – 14.3–14.5 pH and [H⁺] relationships- Study Notes -International A Level (IAL) Chemistry (YCH11) – per latest Syllabus.
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Edexcel International A Level (IAL) Chemistry (YCH11) -Concise Summary Notes- All Topics
14.3 Definition of pH
The pH scale is used to measure the acidity or alkalinity of a solution. It is based on the concentration of hydrogen ions present in aqueous solutions.
Definition
\( \mathrm{pH = -\log_{10}[H^+(aq)]} \)
- \( \mathrm{[H^+]} \) is the concentration of hydrogen ions in \( \mathrm{mol\ dm^{-3}} \).
- pH is a logarithmic scale.
Key Points
- Lower pH → higher \( \mathrm{[H^+]} \) → more acidic.
- Higher pH → lower \( \mathrm{[H^+]} \) → more alkaline.
- A change of 1 pH unit corresponds to a 10× change in \( \mathrm{[H^+]} \).
pH Scale (Typical Values)
- pH < 7 → acidic solution
- pH = 7 → neutral solution
- pH > 7 → alkaline solution
(Note: These values apply at approximately \( \mathrm{298\ K} \)).
Important Notes
- pH has no units because it is a logarithmic value.
- It is strictly defined using \( \mathrm{[H^+]} \), though in reality it relates to activity.
- Applies only to aqueous solutions.
Key Features
- pH measures acidity using hydrogen ion concentration.
- Defined as negative logarithm of \( \mathrm{[H^+]} \).
- Logarithmic scale → large changes in concentration reflected in small pH changes.
Example 1:
Define pH and explain what a decrease of 2 pH units means in terms of \( \mathrm{[H^+]} \).
▶️ Answer/Explanation
pH is defined as \( \mathrm{-\log_{10}[H^+]} \).
A decrease of 2 pH units means the hydrogen ion concentration increases by \( \mathrm{10^2 = 100} \) times.
Example 2:
Calculate the pH of a solution with \( \mathrm{[H^+] = 1.0 \times 10^{-3}\ mol\ dm^{-3}} \).
▶️ Answer/Explanation
\( \mathrm{pH = -\log_{10}(1.0 \times 10^{-3})} \)
\( \mathrm{pH = 3} \)
Therefore, the solution is acidic.
14.4 Calculating pH from Hydrogen Ion Concentration
The pH of a solution can be calculated directly from the hydrogen ion concentration using the logarithmic relationship. This is a key quantitative skill in acid–base chemistry.
Formula
\( \mathrm{pH = -\log_{10}[H^+]} \)
- \( \mathrm{[H^+]} \) must be in \( \mathrm{mol\ dm^{-3}} \).
- Use base-10 logarithm.
Method
- Write down the value of \( \mathrm{[H^+]} \).
- Substitute into \( \mathrm{pH = -\log_{10}[H^+]} \).
- Calculate using a calculator.
- Give answer to appropriate significant figures (usually 2–3 decimal places).
Special Cases
- If \( \mathrm{[H^+] = 1.0 \times 10^{-x}} \), then:
\( \mathrm{pH = x} \)
(only works when coefficient is exactly 1.0)
Key Features
- pH is inversely related to \( \mathrm{[H^+]} \).
- Logarithmic scale → large concentration changes give small pH changes.
- No units for pH.
Example 1:
Calculate the pH of a solution where \( \mathrm{[H^+] = 2.5 \times 10^{-4}\ mol\ dm^{-3}} \).
▶️ Answer/Explanation
\( \mathrm{pH = -\log_{10}(2.5 \times 10^{-4})} \)
\( \mathrm{pH = -( \log_{10}2.5 + \log_{10}10^{-4})} \)
\( \mathrm{pH = -(0.398 – 4)} \)
\( \mathrm{pH = 3.60} \)
Example 2:
A solution has \( \mathrm{[H^+] = 3.2 \times 10^{-2}\ mol\ dm^{-3}} \). Determine its pH and comment on its nature.
▶️ Answer/Explanation
\( \mathrm{pH = -\log_{10}(3.2 \times 10^{-2})} \)
\( \mathrm{pH = -( \log_{10}3.2 – 2)} \)
\( \mathrm{pH = -(0.505 – 2)} = 1.50 \)
The pH is very low, so the solution is strongly acidic.
14.5 Calculating Hydrogen Ion Concentration from pH
The hydrogen ion concentration of a solution can be calculated from its pH by rearranging the pH definition. This is the reverse of calculating pH from \( \mathrm{[H^+]} \).
Formula
\( \mathrm{[H^+] = 10^{-pH}} \)
- \( \mathrm{[H^+]} \) is in \( \mathrm{mol\ dm^{-3}} \).
- Use powers of 10 (antilog).
Method
- Write the given pH value.
- Substitute into \( \mathrm{[H^+] = 10^{-pH}} \).
- Calculate using a calculator (antilog function).
- Express answer in standard form with correct units.
Key Features
- Lower pH → higher \( \mathrm{[H^+]} \).
- Each pH unit change corresponds to a ×10 change in \( \mathrm{[H^+]} \).
- Answer must always be in \( \mathrm{mol\ dm^{-3}} \).
Example 1:
Calculate the hydrogen ion concentration of a solution with pH = 3.25.
▶️ Answer/Explanation
\( \mathrm{[H^+] = 10^{-3.25}} \)
\( \mathrm{[H^+] = 5.62 \times 10^{-4}\ mol\ dm^{-3}} \)
Example 2:
A solution has pH = 9.40. Calculate \( \mathrm{[H^+]} \) and comment on the nature of the solution.
▶️ Answer/Explanation
\( \mathrm{[H^+] = 10^{-9.40}} \)
\( \mathrm{[H^+] = 3.98 \times 10^{-10}\ mol\ dm^{-3}} \)
Since pH > 7, the solution is alkaline.