pH of acids

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If I dilute a 1M concentration of HCl what will happen to its pH

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[+1] - Quincy - 10/10/2006nothing. It is just more dilute now, but the pH is unaffected.
[+1] - McTag - 10/10/2006Is that right? I'd have thought the pH would be changed?
[+1] - spidergal - 10/10/2006I won't give a direct answer. Use your brain. pH is a measure of hydrogen ion (H+) concentration. (pH = - log [H+]) If you add water to Hydrochloric acid, how will that affect the H+...
[+1] - Gouki - 10/10/2006Think about what concentration is equal to: c = n/v where c is concentration, n is the number of moles of the solute and v is the volume. If you 'dilute' a solution, it means you're adding...
[+1] - patiodog - 10/10/2006In an on-the-fly, clinical kind of way, I've come to think of pH as a scale of [OH-] in the solution -- which may not always be right, I guess, but it's close enough for anybody's purposes. ...
[+1] - Quincy - 10/12/2006wouldn't the pH of the solution change, the acid's pH is always the same, or am I missing something...
[+1] - spidergal - 10/12/2006The poster didn't make it clear in the first message. Of course, pH of an acid is constant. Ugh, and he/she, I suppose, is not going to even turn up!
[+1] - patiodog - 10/12/2006No, the pH WILL change! 1M HCl just means that there are 1 mole of H+ ions dissolved per liter of solution. If you dilute 1 liter to, say, 2 liters by adding 1 liter of water, the pH will rise...
[+1] - spendius - 10/12/2006Wouldn't 8 pints of John Smith's Extra Smooth be even better?
[+1] - patiodog - 10/12/2006Plus some fish and chips, and make sure the vinegar is of the malt variety, and I'd be a happy puppy. (Mmmmm, pitchers of ale and fish and chips back at the Pacific Inn... What in the hell...

nothing. It is just more dilute now, but the pH is unaffected.

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Is that right? I'd have thought the pH would be changed?

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I won't give a direct answer. Use your brain.

pH is a measure of hydrogen ion (H+) concentration. (pH = - log [H+])

If you add water to Hydrochloric acid, how will that affect the H+ concentration of the solution? Will it increase, decrease or remain unaffected? pH will vary accordingly.

If H+ concentration increases, pH will be reduced i.e it it were 4 it will jump back to 2 . Hence, the solution will become more acidic. (This is just an example and not necessarily true for your situation)

Now, you should be able to figure it out.

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Think about what concentration is equal to:

c = n/v where c is concentration, n is the number of moles of the solute and v is the volume.

If you 'dilute' a solution, it means you're adding to the volume of the solution, so v increases, n stays the same. (That means n is divided by a larger number) c inevitably decreases.

pH = -log[H+], if c decreases, pH must increase, which indicates that the solution is turning LESS acidic.

Hope this helps.

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In an on-the-fly, clinical kind of way, I've come to think of pH as a scale of [OH-] in the solution -- which may not always be right, I guess, but it's close enough for anybody's purposes.

Course, then you have to think about pH in terms of the equilibrium,

H+ + OH- <--> H2O

but I always find that easier to think about than the algebra of it.

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wouldn't the pH of the solution change, the acid's pH is always the same, or am I missing something...

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The poster didn't make it clear in the first message.

Of course, pH of an acid is constant.

Ugh, and he/she, I suppose, is not going to even turn up!

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No, the pH WILL change!

1M HCl just means that there are 1 mole of H+ ions dissolved per liter of solution. If you dilute 1 liter to, say, 2 liters by adding 1 liter of water, the pH will rise (the solution will be less acidic). 'Tis the nature of dilution. Would you rather have to drink a cup of vinegar straight from the bottle, or would you rather be allowed to dilute it with a gallon of water first? (Assuming that not drinking is not an option, of course...)

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Wouldn't 8 pints of John Smith's Extra Smooth be even better?

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Plus some fish and chips, and make sure the vinegar is of the malt variety, and I'd be a happy puppy.

(Mmmmm, pitchers of ale and fish and chips back at the Pacific Inn... What in the hell am I doing in America's heartland?)

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Re: pH of acids

lledbrook wrote:

If I dilute a 1M concentration of HCl what will happen to its pH

An important piece of information, you've not posted is the extent of the dilution. If you mix 1 ml of the acid with 1 ml of water, the pH will hardly change at all, as water does not buffer the acid.
Under these conditions, the pH will be 1-2.

Now, if you take 1 ml of the acid and drop it into the ocean, the pH of the ocean will NOT become 1-2, will it? O f course not.
The pH of the whole ocean will not change, after the acid mixes with the ocean water.

So, before you can answer your posted question, you must post the vol of the water and HCl that are involved.

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Quincy wrote:

wouldn't the pH of the solution change, the acid's pH is always the same, or am I missing something...

You're missing something!

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spidergal wrote:

The poster didn't make it clear in the first message.

Of course, pH of an acid is constant.

Ugh, and he/she, I suppose, is not going to even turn up!

No the pH of the acid is not a constant. What's a constant is the pKa of the acid!

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Re: pH of acids

Miller wrote:

lledbrook wrote:
If I dilute a 1M concentration of HCl what will happen to its pH

An important piece of information, you've not posted is the extent of the dilution. If you mix 1 ml of the acid with 1 ml of water, the pH will hardly change at all, as water does not buffer the acid.
Under these conditions, the pH will be 1-2.

Now, if you take 1 ml of the acid and drop it into the ocean, the pH of the ocean will NOT become 1-2, will it? O f course not.
The pH of the whole ocean will not change, after the acid mixes with the ocean water.

So, before you can answer your posted question, you must post the vol of the water and HCl that are involved.

1M HCl has a pH of 0: -log(1)=0
0.5M HCl (what you get if you dilute 1M HCl with an equal volume of distille water) has a pH of roughly 0.3: -log(0.5)=0.3 (or should; on a logarithmic scale, a change of 0.3 is a roughly two-fold change in magnitude on a linear scale)

Doesn't matter how disparate the volumes are, you still know what the direction of the change is (even for 1 ml dropped into the ocean), which is all the question is asking for.

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Re: pH of acids
BM

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Miller wrote:

spidergal wrote:
The poster didn't make it clear in the first message.

Of course, pH of an acid is constant.

Ugh, and he/she, I suppose, is not going to even turn up!

No the pH of the acid is not a constant. What's a constant is the pKa of the acid!

Yeah, pKa. Rolling Eyes

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pH of acids

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