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Understanding the pH Scale and Strong Acids - Chapter 20.2

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. HCl

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!

20.2- the pH scale and strong acids
Key Facts:
Notes:
Strong and weak acids
-strong acid completely dissociates in
aqueous solution
e.g. HCl

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 = -logH+(aq)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)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!

20.2- the pH scale and strong acids
Key Facts:
Notes:
Strong and weak acids
-strong acid completely dissociates in
aqueous solution
e.g. HCl

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+H⁺ = HClHCl = 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+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+H⁺ always equals acidacid - this makes calculations much simpler!

20.2- the pH scale and strong acids
Key Facts:
Notes:
Strong and weak acids
-strong acid completely dissociates in
aqueous solution
e.g. HCl

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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This apps acc the goat. I find revision so boring but this app makes it so easy to organize it all and then you can ask the freeeee ai to test yourself so good and you can easily upload your own stuff. highly recommend as someone taking mocks now

Paul T

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The app is very easy to use and well designed. I have found everything I was looking for so far and have been able to learn a lot from the presentations! I will definitely use the app for a class assignment! And of course it also helps a lot as an inspiration.

Stefan S

iOS user

This app is really great. There are so many study notes and help [...]. My problem subject is French, for example, and the app has so many options for help. Thanks to this app, I have improved my French. I would recommend it to anyone.

Samantha Klich

Android user

Wow, I am really amazed. I just tried the app because I've seen it advertised many times and was absolutely stunned. This app is THE HELP you want for school and above all, it offers so many things, such as workouts and fact sheets, which have been VERY helpful to me personally.

Anna

iOS user

Best app on earth! no words because it’s too good

Thomas R

iOS user

Just amazing. Let's me revise 10x better, this app is a quick 10/10. I highly recommend it to anyone. I can watch and search for notes. I can save them in the subject folder. I can revise it any time when I come back. If you haven't tried this app, you're really missing out.

Basil

Android user

This app has made me feel so much more confident in my exam prep, not only through boosting my own self confidence through the features that allow you to connect with others and feel less alone, but also through the way the app itself is centred around making you feel better. It is easy to navigate, fun to use, and helpful to anyone struggling in absolutely any way.

David K

iOS user

The app's just great! All I have to do is enter the topic in the search bar and I get the response real fast. I don't have to watch 10 YouTube videos to understand something, so I'm saving my time. Highly recommended!

Sudenaz Ocak

Android user

In school I was really bad at maths but thanks to the app, I am doing better now. I am so grateful that you made the app.

Greenlight Bonnie

Android user

very reliable app to help and grow your ideas of Maths, English and other related topics in your works. please use this app if your struggling in areas, this app is key for that. wish I'd of done a review before. and it's also free so don't worry about that.

Rohan U

Android user

I know a lot of apps use fake accounts to boost their reviews but this app deserves it all. Originally I was getting 4 in my English exams and this time I got a grade 7. I didn’t even know about this app three days until the exam and it has helped A LOT. Please actually trust me and use it as I’m sure you too will see developments.

Xander S

iOS user

THE QUIZES AND FLASHCARDS ARE SO USEFUL AND I LOVE THE SCHOOLGPT. IT ALSO IS LITREALLY LIKE CHATGPT BUT SMARTER!! HELPED ME WITH MY MASCARA PROBLEMS TOO!! AS WELL AS MY REAL SUBJECTS ! DUHHH 😍😁😲🤑💗✨🎀😮

Elisha

iOS user

This apps acc the goat. I find revision so boring but this app makes it so easy to organize it all and then you can ask the freeeee ai to test yourself so good and you can easily upload your own stuff. highly recommend as someone taking mocks now

Paul T

iOS user

 

Chemistry

270

11 Dec 2025

4 pages

Understanding the pH Scale and Strong Acids - Chapter 20.2

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.

20.2- the pH scale and strong acids
Key Facts:
Notes:
Strong and weak acids
-strong acid completely dissociates in
aqueous solution
e.g. HCl

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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!

20.2- the pH scale and strong acids
Key Facts:
Notes:
Strong and weak acids
-strong acid completely dissociates in
aqueous solution
e.g. HCl

Sign up to see the contentIt's free!

Access to all documents

Improve your grades

Join milions of students

By signing up you accept Terms of Service and Privacy Policy

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 = -logH+(aq)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)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!

20.2- the pH scale and strong acids
Key Facts:
Notes:
Strong and weak acids
-strong acid completely dissociates in
aqueous solution
e.g. HCl

Sign up to see the contentIt's free!

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By signing up you accept Terms of Service and Privacy Policy

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+H⁺ = HClHCl = 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+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+H⁺ always equals acidacid - this makes calculations much simpler!

20.2- the pH scale and strong acids
Key Facts:
Notes:
Strong and weak acids
-strong acid completely dissociates in
aqueous solution
e.g. HCl

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By signing up you accept Terms of Service and Privacy Policy

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!

We thought you’d never ask...

What is the Knowunity AI companion?

Our AI Companion is a student-focused AI tool that offers more than just answers. Built on millions of Knowunity resources, it provides relevant information, personalised study plans, quizzes, and content directly in the chat, adapting to your individual learning journey.

Where can I download the Knowunity app?

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Is Knowunity really free of charge?

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4.8/5

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The app is very easy to use and well designed. I have found everything I was looking for so far and have been able to learn a lot from the presentations! I will definitely use the app for a class assignment! And of course it also helps a lot as an inspiration.

Stefan S

iOS user

This app is really great. There are so many study notes and help [...]. My problem subject is French, for example, and the app has so many options for help. Thanks to this app, I have improved my French. I would recommend it to anyone.

Samantha Klich

Android user

Wow, I am really amazed. I just tried the app because I've seen it advertised many times and was absolutely stunned. This app is THE HELP you want for school and above all, it offers so many things, such as workouts and fact sheets, which have been VERY helpful to me personally.

Anna

iOS user

Best app on earth! no words because it’s too good

Thomas R

iOS user

Just amazing. Let's me revise 10x better, this app is a quick 10/10. I highly recommend it to anyone. I can watch and search for notes. I can save them in the subject folder. I can revise it any time when I come back. If you haven't tried this app, you're really missing out.

Basil

Android user

This app has made me feel so much more confident in my exam prep, not only through boosting my own self confidence through the features that allow you to connect with others and feel less alone, but also through the way the app itself is centred around making you feel better. It is easy to navigate, fun to use, and helpful to anyone struggling in absolutely any way.

David K

iOS user

The app's just great! All I have to do is enter the topic in the search bar and I get the response real fast. I don't have to watch 10 YouTube videos to understand something, so I'm saving my time. Highly recommended!

Sudenaz Ocak

Android user

In school I was really bad at maths but thanks to the app, I am doing better now. I am so grateful that you made the app.

Greenlight Bonnie

Android user

very reliable app to help and grow your ideas of Maths, English and other related topics in your works. please use this app if your struggling in areas, this app is key for that. wish I'd of done a review before. and it's also free so don't worry about that.

Rohan U

Android user

I know a lot of apps use fake accounts to boost their reviews but this app deserves it all. Originally I was getting 4 in my English exams and this time I got a grade 7. I didn’t even know about this app three days until the exam and it has helped A LOT. Please actually trust me and use it as I’m sure you too will see developments.

Xander S

iOS user

THE QUIZES AND FLASHCARDS ARE SO USEFUL AND I LOVE THE SCHOOLGPT. IT ALSO IS LITREALLY LIKE CHATGPT BUT SMARTER!! HELPED ME WITH MY MASCARA PROBLEMS TOO!! AS WELL AS MY REAL SUBJECTS ! DUHHH 😍😁😲🤑💗✨🎀😮

Elisha

iOS user

This apps acc the goat. I find revision so boring but this app makes it so easy to organize it all and then you can ask the freeeee ai to test yourself so good and you can easily upload your own stuff. highly recommend as someone taking mocks now

Paul T

iOS user

The app is very easy to use and well designed. I have found everything I was looking for so far and have been able to learn a lot from the presentations! I will definitely use the app for a class assignment! And of course it also helps a lot as an inspiration.

Stefan S

iOS user

This app is really great. There are so many study notes and help [...]. My problem subject is French, for example, and the app has so many options for help. Thanks to this app, I have improved my French. I would recommend it to anyone.

Samantha Klich

Android user

Wow, I am really amazed. I just tried the app because I've seen it advertised many times and was absolutely stunned. This app is THE HELP you want for school and above all, it offers so many things, such as workouts and fact sheets, which have been VERY helpful to me personally.

Anna

iOS user

Best app on earth! no words because it’s too good

Thomas R

iOS user

Just amazing. Let's me revise 10x better, this app is a quick 10/10. I highly recommend it to anyone. I can watch and search for notes. I can save them in the subject folder. I can revise it any time when I come back. If you haven't tried this app, you're really missing out.

Basil

Android user

This app has made me feel so much more confident in my exam prep, not only through boosting my own self confidence through the features that allow you to connect with others and feel less alone, but also through the way the app itself is centred around making you feel better. It is easy to navigate, fun to use, and helpful to anyone struggling in absolutely any way.

David K

iOS user

The app's just great! All I have to do is enter the topic in the search bar and I get the response real fast. I don't have to watch 10 YouTube videos to understand something, so I'm saving my time. Highly recommended!

Sudenaz Ocak

Android user

In school I was really bad at maths but thanks to the app, I am doing better now. I am so grateful that you made the app.

Greenlight Bonnie

Android user

very reliable app to help and grow your ideas of Maths, English and other related topics in your works. please use this app if your struggling in areas, this app is key for that. wish I'd of done a review before. and it's also free so don't worry about that.

Rohan U

Android user

I know a lot of apps use fake accounts to boost their reviews but this app deserves it all. Originally I was getting 4 in my English exams and this time I got a grade 7. I didn’t even know about this app three days until the exam and it has helped A LOT. Please actually trust me and use it as I’m sure you too will see developments.

Xander S

iOS user

THE QUIZES AND FLASHCARDS ARE SO USEFUL AND I LOVE THE SCHOOLGPT. IT ALSO IS LITREALLY LIKE CHATGPT BUT SMARTER!! HELPED ME WITH MY MASCARA PROBLEMS TOO!! AS WELL AS MY REAL SUBJECTS ! DUHHH 😍😁😲🤑💗✨🎀😮

Elisha

iOS user

This apps acc the goat. I find revision so boring but this app makes it so easy to organize it all and then you can ask the freeeee ai to test yourself so good and you can easily upload your own stuff. highly recommend as someone taking mocks now

Paul T

iOS user