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Haitham Wahid

Chemistry help!

Hi guys. 

 

I just want to show you a multiple choice question that I think has two answers, though I am not 100%, so I want to have your opinions on this one.  

post-131733-0-62062600-1427488916_thumb.

Edited by Haitham Wahid

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Hi guys. 

 

I just want to show you a multiple choice question that I think has two answers, though I am not 100%, so I want to have your opinions on this one.  

 

C is the only correct answer to this question.

 

I guess that you think A could also be the right answer? Well no, it couldn’t! This is because the strength of an acid only depends on the acid’s ability to dissociate in water. Strong acid dissociates almost entirely; on the other hand, weak acid has little dissociation (approx. 1%).

 

Despite all of that, it is usually the case that low pH indicates stronger acid. However, low pH not the necessary requirement. HCl, for example, can be diluted to the pH of 5 if you want. Remember that the question uses the word “must”, so C is the only right answer, because C is the necessary requirement while A is not.

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Hi guys. 

 

I just want to show you a multiple choice question that I think has two answers, though I am not 100%, so I want to have your opinions on this one.  

 

C is the only correct answer to this question.

 

I guess that you think A could also be the right answer? Well no, it couldn’t! This is because the strength of an acid only depends on the acid’s ability to dissociate in water. Strong acid dissociates almost entirely; on the other hand, weak acid has little dissociation (approx. 1%).

 

Despite all of that, it is usually the case that low pH indicates stronger acid. However, low pH not the necessary requirement. HCl, for example, can be diluted to the pH of 5 if you want. Remember that the question uses the word “mustâ€, so C is the only right answer, because C is the necessary requirement while A is not.

 

 

That was my choice but A was a bit of distracting. But are you saying that an acid can actually have a ph higher then 7 if it was extremely diluted ? 

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That was my choice but A was a bit of distracting. But are you saying that an acid can actually have a ph higher then 7 if it was extremely diluted ?

 

If you look closely at the equation for pH (i.e. pH = -log[H+]), you will see that pH only depends on the concentration of hydrogen ions. However, the level of acidity/alkalinity of a solution does not only depend on the concentration of hydrogen ions (i.e. pH), but also depends on the concentration of the OH–ions (i.e. pOH). See the explanation below:

 

post-115475-0-07620200-1427632261_thumb.

 

Technically, you could dilute a solution of HCl to the pH extremely close to 7 by just adding more water into it. This is because if you add more water, the concentration of H+ will decrease, thus increasing the pH level. However, notice that the pH will never grow higher than pH 7 because the water itself has the pH of 7. So if you keep adding more water, the water will become the dominant part of the solution, thus the pH will just keep approaching the limit of 7.

 

I know all of these sound confusing & counter-intuitive, because students are often taught in secondary schools that pH lower than 7 will automatically indicate an acid. This is not entirely correct in actual chemistry, even though it's usually the case.

Feel free to ask if you have further questions!

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That was my choice but A was a bit of distracting. But are you saying that an acid can actually have a ph higher then 7 if it was extremely diluted ?

 

If you look closely at the equation for pH (i.e. pH = -log[H+]), you will see that pH only depends on the concentration of hydrogen ions. However, the level of acidity/alkalinity of a solution does not only depend on the concentration of hydrogen ions (i.e. pH), but also depends on the concentration of the OH–ions (i.e. pOH). See the explanation below:

 

attachicon.gifUntitled.png

 

Technically, you could dilute a solution of HCl to the pH extremely close to 7 by just adding more water into it. This is because if you add more water, the concentration of H+ will decrease, thus increasing the pH level. However, notice that the pH will never grow higher than pH 7 because the water itself has the pH of 7. So if you keep adding more water, the water will become the dominant part of the solution, thus the pH will just keep approaching the limit of 7.

 

I know all of these sound confusing & counter-intuitive, because students are often taught in secondary schools that pH lower than 7 will automatically indicate an acid. This is not entirely correct in actual chemistry, even though it's usually the case.

Feel free to ask if you have further questions!

 

Oh right I understand! Thanks a lot for the help! 

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