6.2pH Value
pH is a logarithmic scale ranging from 0 to 14 used to indicate the acidity or alkalinity of an aqueous solution. In chemistry, pH directly measures the concentration of hydrogen ions, \(\text{H}^+\), in an aqueous solution. Understanding pH helps determine the nature of acidic and alkaline substances in everyday life, from maintaining healthy water conditions for clownfish (pH 8.0 to 8.3) to evaluating common household items.
Learning Objectives
- State the meaning of pH and its uses.
- Calculate the pH values of acids and alkalis.
- Investigate the relationship between pH value and the concentration of hydrogen ions and hydroxide ions through experiment.
The pH Scale and Everyday Substances
Visualise the relationship between pH values, degree of acidity/alkalinity, and typical household examples.
Brief Explanation
Understand the meaning of pH and how to calculate it.
Meaning and Scale of pH
pH is a logarithmic measure of the concentration of hydrogen ions (\(\text{H}^+\)) in an aqueous solution.
\(\text{pH} = -\log[\text{H}^+]\)
where \(\log\) is logarithm of base 10 and \([\text{H}^+]\) is the concentration of hydrogen ions in \(\text{mol dm}^{-3}\).
The pH scale ranges from 0 to 14.
- \(\text{pH} < 7\): Acidic solution (more acidic towards 0).
- \(\text{pH} = 7\): Neutral solution.
- \(\text{pH} > 7\): Alkaline solution (more alkaline towards 14).
Calculation of pOH and pH for Alkalis
\(\text{pOH} = -\log[\text{OH}^-]\)
where \([\text{OH}^-]\) represents the concentration of hydroxide ions in \(\text{mol dm}^{-3}\).
\(\text{pH} + \text{pOH} = 14\)
\(\text{pH} = 14 - \text{pOH}\)
| Worked Example | Calculation |
|---|---|
| Nitric acid, \(\text{HNO}_3\), with \(0.5\text{ mol dm}^{-3}\) of \(\text{H}^+\) | \(\text{pH} = -\log[0.5] = -(-0.301) = 0.301 \approx 0.3\) |
| Hydrochloric acid, \(\text{HCl}\), with \(\text{pH} = 2.0\) | \(2.0 = -\log[\text{H}^+] \implies [\text{H}^+] = 10^{-2.0} = 0.01\text{ mol dm}^{-3}\) |
| Sodium hydroxide, \(\text{NaOH}\), with \(0.1\text{ mol dm}^{-3}\) \(\text{OH}^-\) | \(\text{pOH} = -\log[0.1] = -(-1) = 1.0\) |
| Potassium hydroxide, \(\text{KOH}\), with \(0.01\text{ mol dm}^{-3}\) \(\text{OH}^-\) | \(\text{pOH} = -\log[0.01] = 2.0\), \(\text{pH} = 14.0 - 2.0 = 12.0\) |
| Lithium hydroxide, \(\text{LiOH}\), with \(\text{pH} = 12.0\) | \(\text{pOH} = 14.0 - 12.0 = 2.0\), \([\text{OH}^-] = 10^{-2.0} = 0.01\text{ mol dm}^{-3}\) |
Relationship Between Ion Concentration and pH
Acids: When the concentration of acid increases, more acid molecules ionise to produce \(\text{H}^+\) ions. The higher the concentration of hydrogen ions, \([\text{H}^+]\), the lower the pH value. Acidity increases when the pH value decreases. When an acidic solution is diluted, \([\text{H}^+]\) decreases, pH value increases towards 7, and the degree of acidity decreases.
Alkalis: The higher the concentration of hydroxide ions, \([\text{OH}^-]\), the higher the pH value. Alkalinity increases when the pH value increases. When an alkaline solution is diluted, \([\text{OH}^-]\) decreases, pH value decreases towards 7, and the degree of alkalinity decreases.
Significant Figures and Decimal Places Rule
The decimal place of pH is related to the significant figures in the given concentration of hydrogen ions. If the value of the given concentration has two significant figures, the pH value should be rounded to two decimal places.
Measuring Tools: Universal indicator, pH meter, or pH paper.
Real-life Context: Clownfish rearing requires water with specific pH values between 8.0 to 8.3 to remain healthy.
Try These Questions First
Answer mentally, then check the explanation.
What is the pH value of a hydrochloric acid solution with a hydrogen ion concentration of \(0.001\text{ mol dm}^{-3}\)?
If the concentration of hydrogen ions in an acid increases, what happens to its pH value and degree of acidity?
Calcium hydroxide solution, \(\text{Ca(OH)}_2\), has a concentration of \(0.05\text{ mol dm}^{-3}\). Since each \(\text{Ca(OH)}_2\) molecule produces two \(\text{OH}^-\) ions, its \([\text{OH}^-]\) is \(2 \times 0.05 = 0.1\text{ mol dm}^{-3}\). What is its pH value?
Common Mistakes
Avoid these errors when working with pH and pOH.
Common Misconceptions
Compare each misconception with the correct fact.
A solution with pH 6 is alkaline because it is close to 7.
Any solution with a pH below 7 is acidic. Only solutions with \(\text{pH} > 7\) are alkaline, and \(\text{pH} = 7\) is neutral.
Diluting an acid with water makes it turn alkaline.
Diluting an acid decreases \([\text{H}^+]\), which increases the pH value, but it only approaches 7 (neutrality); it cannot become alkaline (\(\text{pH} > 7\)).
The pH formula uses natural logarithm (\(\ln\)).
The formula strictly uses common logarithm of base 10 (\(\log_{10}\)).
Summary
Key facts to remember.
- \(\text{pH}\) measures \([\text{H}^+]\): \(\text{pH} = -\log[\text{H}^+]\).
- \(\text{pOH}\) measures \([\text{OH}^-]\): \(\text{pOH} = -\log[\text{OH}^-]\).
- Total relationship: \(\text{pH} + \text{pOH} = 14\).
- \([\text{H}^+] \uparrow \implies \text{pH} \downarrow \implies \text{Acidity} \uparrow\).
- \([\text{OH}^-] \uparrow \implies \text{pH} \uparrow \implies \text{Alkalinity} \uparrow\).
- \(\text{pH} < 7\) is acidic, \(\text{pH} = 7\) is neutral, and \(\text{pH} > 7\) is alkaline.
Quick Activity
Complete all parts before checking your score.
A. Objective Quiz
What is the pH value of a solution of nitric acid, \(\text{HNO}_3\), containing a hydrogen ion concentration of \(0.01\text{ mol dm}^{-3}\)?
Which statement correctly describes what happens when the concentration of hydroxide ions, \(\text{OH}^-\), in an alkaline solution decreases?
B. Fill in the Blanks
In chemistry, pH is a logarithmic measure of the concentration of ________ ions in an aqueous solution.
Given the relationship between pH and pOH, the sum of pH and pOH values is equal to ________.
C. Match the Substance with Its pH Value
Drag each substance on desktop. On mobile, tap a substance and then tap the matching box.
Substances
Gastric juice Pure water Blood BleachpH Values
Keywords
Tap or hover over a term to read its meaning.
