Understanding Acid Concentration: Factors of Quantity and Ionization

[Yashbhatt]

The concentrations of an acid depends on ________________.
(a) Quantity
(b) Ionization
(c) Both

It depends on what exactly we mean by concentrated? I have read a statement like hydrochloric acid having concentration 35-38% is considered concentrated while sulphuric acid having concentration 90% is considered concentrated. So, I am quite confused.

 

[Giant]

Informally, concentration is amount of acid per unit volume of water.

Strength of acid can be determined by how much H+ ions it gives. If H+ ions per molecule is known then moles per unit volume can give you the concentration(molarity=moles/metre^3). H+ ions can b found out by knowing the dissociation of that particular acid. So more moles of acid per unit volume more is the molarity. There is a better yet system called normality. It directly measures H+ ion concentration (only for acids). 1 Molar sulfuric acid (H2SO4) is 2 Normal because each mole of sulfuric acid gives 2 moles of H+ ions.
For HCl 1M=1N meaning it gives 1 mole H+ ions for 1 mole HCl. This is the background you need to solve that question.

I have read a statement like hydrochloric acid having concentration 35-38% is considered concentrated while sulphuric acid having concentration 90% is considered concentrated.

Read the whole book again before attempting the questions.

 

[Borek]

Informally, concentration is amount of acid per unit volume of water.

What is informal about this definition?

Strength of acid can be determined by how much H+ ions it gives. If H+ ions per molecule is known then moles per unit volume can give you the concentration(molarity=moles/metre^3). H+ ions can b found out by knowing the dissociation of that particular acid. So more moles of acid per unit volume more is the molarity. There is a better yet system called normality. It directly measures H+ ion concentration (only for acids). 1 Molar sulfuric acid (H2SO4) is 2 Normal because each mole of sulfuric acid gives 2 moles of H+ ions.
For HCl 1M=1N meaning it gives 1 mole H+ ions for 1 mole HCl. This is the background you need to solve that question.

Question doesn't ask about strength, it asks about concentration.

 

[Borek]

It depends on what exactly we mean by concentrated? I have read a statement like hydrochloric acid having concentration 35-38% is considered concentrated while sulphuric acid having concentration 90% is considered concentrated. So, I am quite confused.

I am not surprised by your confusion. This is rather convoluted, and what is considered concentrated depends on a mix of practical and thermodynamical reasons.

At STP sulfuric acid is a liquid, so it is not particularly difficult to prepare 100% sulfuric acid.

At STP HCl is a gas, and its solubility in water is limited. Around 38% is a maximum concentration that can be prepared without using high pressure of HCl. Any more concentrated solution left in contact with air will emit HCl fumes till the concentration goes down to about 35-38%. As it is not possible to have more concentrated HCl solution at STP, this solution is considered concentrated.

In the case of nitric acid situation looks a little bit different. Pure (100%) nitric acid is a liquid at STP. However, when concentrating nitric acid by distillation (which is a standard approach in industry) its concentration gets up to 68% but not higher (that's because it creates azeotrope at that concentration). It is possible to prepare more concentrated nitric acid by other means, but the price goes up, and 68% acid is already concentrated enough for most applications - so it is considered "concentrated".

 

[Giant]

What is informal about this definition?

Different concepts exist to define concentration. I was not specific enough. As you said some acids can't be be prepared to 100% concentration. There are lots of factors to be taken into consideration. I din't mention (know) them all. So I said informally.

Question doesn't ask about strength, it asks about concentration.

Okay, that's correct I'm sorry.

 

[Yashbhatt]

I am not surprised by your confusion. This is rather convoluted, and what is considered concentrated depends on a mix of practical and thermodynamical reasons.

At STP sulfuric acid is a liquid, so it is not particularly difficult to prepare 100% sulfuric acid.

At STP HCl is a gas, and its solubility in water is limited. Around 38% is a maximum concentration that can be prepared without using high pressure of HCl. Any more concentrated solution left in contact with air will emit HCl fumes till the concentration goes down to about 35-38%. As it is not possible to have more concentrated HCl solution at STP, this solution is considered concentrated.

In the case of nitric acid situation looks a little bit different. Pure (100%) nitric acid is a liquid at STP. However, when concentrating nitric acid by distillation (which is a standard approach in industry) its concentration gets up to 68% but not higher (that's because it creates azeotrope at that concentration). It is possible to prepare more concentrated nitric acid by other means, but the price goes up, and 68% acid is already concentrated enough for most applications - so it is considered "concentrated".

Ok. Got it. So, concentration has got nothing to do with ionisation?

 

[Borek]

So, concentration has got nothing to do with ionisation?

No, these are separate concepts.

More precisely, for a given acid changing concentration changes dissociation level (ionization). Typically the more diluted the acid, the more ionized it is.

 

[Yashbhatt]

But what if we look at it this way. Suppose we have 350 ml of HCl and 350 ml acetic acid. Now, we dissolve both of them in separate containers containing 1000 ml water each. So, won't the solution containing Hcl be considered "concentrated" and the one containing acetic acid considered "dilute"?

 

[Borek]

Suppose we have 350 ml of HCl and 350 ml acetic acid.

What are their initial concentrations?

Note that "concentrated" is not precisely defined, and the same solution can be sometimes called diluted and sometimes concentrated, depending on the context.

 

[Yashbhatt]

Well, for simplicity assume their initial concentrations(here I mean based on quantity) to be equal.

 

[Borek]

Then their final concentrations will be equal as well.

 

[Yashbhatt]

OK. Now, I got the exact meaning of concentration. But, then why do we sometimes refer to solution with more ions as "concentrated" and the one with less ions as "dilute"? That's just a misconception, right?

 

[Borek]

Something is wrong. Let's say we have NaCl solution. NaCl is completely dissociated into ions. Let's say we have 1M solution and 0.01M solution. The first solution is 2M in ions, the second is 0.02M in ions. What is wrong about saying the first one is more concentrated?

 

[Yashbhatt]

That was my mistake. Thanks for clearing my concepts.