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Updated Mar 24, 2026

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4 pages

Understanding the pH Scale and Strong Acids - Chapter 20.2

sheila

@study_withsheila

Understanding acids might seem intimidating, but it's actually quite straightforward... Show more

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20.2- the pH scale and strong acids

Key Facts:

Notes:
Strong and weak acids
-strong acid completely dissociates in
aqueous solution
e.g. H

Strong vs Weak Acids

The main difference between strong and weak acids is how completely they break apart in water. Think of it like sugar dissolving - some substances dissolve completely whilst others only partially dissolve.

Strong acids completely dissociate (split up) in water. For example, hydrochloric acid (HCl) breaks apart entirely: HCl(aq) → H⁺(aq) + Cl⁻(aq). The forward reaction goes to completion - there's no going back.

Weak acids only partially dissociate in water. Ethanoic acid (CH₃COOH) is a perfect example: CH₃COOH(aq) ⇌ H⁺(aq) + CH₃COO⁻(aq). Notice the double arrow - this means the reaction is reversible and doesn't go to completion.

Quick Check: Strong acids = complete dissociation, weak acids = partial dissociation. It's that simple!

Understanding the pH Scale

The pH scale is basically a shortcut for measuring how many H⁺ ions are floating around in your solution. It's much easier than dealing with tiny decimal numbers all the time.

The mathematical expression is: pH = -log[H⁺(aq)]. Don't panic about the maths - it's just a way to convert very small numbers into manageable ones. Always give your pH to 2 decimal places, and remember it has no units.

Here's the key relationship to remember: smaller pH = more H⁺ ions = more acidic. A pH change of just one unit means the H⁺ concentration has changed by 10 times - that's a massive difference!

You can also work backwards using: [H⁺(aq)] = 10⁻ᵖᴴ. This is handy when you know the pH but need the concentration.

Remember: pH 2 is 10 times more acidic than pH 3, and 100 times more acidic than pH 4!

Calculating pH of Strong Acids

Calculating the pH of strong acids is straightforward because they completely dissociate. For monobasic acids (acids that donate one H⁺ ion), the concentration of H⁺ ions equals the concentration of the acid.

Let's work through an example: Calculate the pH of 1.56 × 10⁻² mol dm⁻³ HCl. Since HCl completely dissociates, [H⁺] = [HCl] = 1.56 × 10⁻². Therefore, pH = -log(1.56 × 10⁻²) = 1.81.

Working backwards is just as easy. If HCl has a pH of 3.79, then [H⁺] = 10⁻³·⁷⁹ = 1.62 × 10⁻⁴ mol dm⁻³. Since it's a strong monobasic acid, this is also the concentration of the original acid.

Top Tip: For strong monobasic acids, [H⁺] always equals [acid] - this makes calculations much simpler!

Dilution and pH Changes

When you dilute acids, the pH changes because you're reducing the concentration of H⁺ ions. The key formula is: new concentration = original concentration × (volume of reagent ÷ total volume).

Let's see this in action: 10 cm³ of 0.250 mol dm⁻³ HCl is diluted to 200 cm³ with water. Before dilution, pH = -log(0.25) = 0.60.

After dilution, the new concentration becomes: 0.250 × (10/200) = 0.0125 mol dm⁻³. The new pH = -log(0.0125) = 1.90.

The pH change is 1.90 - 0.60 = 1.3. Notice how diluting the acid made it less acidic (higher pH), which makes perfect sense!

Key Point: Dilution always increases pH (makes solutions less acidic) - you're adding water, not more acid!

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272

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Updated Mar 24, 2026

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4 pages

Understanding the pH Scale and Strong Acids - Chapter 20.2

sheila

@study_withsheila

Understanding acids might seem intimidating, but it's actually quite straightforward once you grasp the basics. This topic covers how acids behave differently in water and introduces the pH scale - a simple way to measure how acidic a solution is.

Sign up to see the contentIt's free!

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Strong vs Weak Acids

Quick Check: Strong acids = complete dissociation, weak acids = partial dissociation. It's that simple!

Sign up to see the contentIt's free!

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Improve your grades

Join milions of students

Understanding the pH Scale

The pH scale is basically a shortcut for measuring how many H⁺ ions are floating around in your solution. It's much easier than dealing with tiny decimal numbers all the time.

You can also work backwards using: [H⁺(aq)] = 10⁻ᵖᴴ. This is handy when you know the pH but need the concentration.

Remember: pH 2 is 10 times more acidic than pH 3, and 100 times more acidic than pH 4!

Sign up to see the contentIt's free!

Access to all documents

Improve your grades

Join milions of students

Calculating pH of Strong Acids

Let's work through an example: Calculate the pH of 1.56 × 10⁻² mol dm⁻³ HCl. Since HCl completely dissociates, [H⁺] = [HCl] = 1.56 × 10⁻². Therefore, pH = -log(1.56 × 10⁻²) = 1.81.

Top Tip: For strong monobasic acids, [H⁺] always equals [acid] - this makes calculations much simpler!

Sign up to see the contentIt's free!

Access to all documents

Improve your grades

Join milions of students

Dilution and pH Changes

Let's see this in action: 10 cm³ of 0.250 mol dm⁻³ HCl is diluted to 200 cm³ with water. Before dilution, pH = -log(0.25) = 0.60.

After dilution, the new concentration becomes: 0.250 × (10/200) = 0.0125 mol dm⁻³. The new pH = -log(0.0125) = 1.90.

The pH change is 1.90 - 0.60 = 1.3. Notice how diluting the acid made it less acidic (higher pH), which makes perfect sense!

Key Point: Dilution always increases pH (makes solutions less acidic) - you're adding water, not more acid!