[smcmanus]

Hi, I am doing a project and need to explain how the pH will change when the % concentration of a chemical changes.  Like if it went from 20% to 50% Nitric Acid, how would you calculate how the pH would change?
Thanks
Sarah

[Borek]

Check these pH calculation lectures.

[smcmanus]

Thanks for the calculator.  The only thing I am having trouble with now is how to go from % to concentration.  Ex. 50% Nitric Acid is what concentration?  
Thanks
Sarah

[Borek]

Thanks for the calculator.  The only thing I am having trouble with now is how to go from % to concentration.  Ex. 50% Nitric Acid is what concentration?  

I am assuming you mean molar concentration. On the same site, concentration lectures:

http://www.chembuddy.com/?left=concentration&right=percentage-to-molarity

Note that you need solution density for conversion - you will find density tables in many handbooks (like CRC).

Once you know how to do the calculation by yourself you may try to do them using CASC (for % to molarity conversion) and BATE (for pH). Trial versions of both programs are also on my site (check the footer).

[smcmanus]

Thanks I finally have it.  What is the convention when dealing with negative pH values?   Do you just make them 0?

[Borek]

No, just left them negative.

[jubba]

You have to remember there are maximum thresholds of solubility for some acids. so there is a maximum concentration

[smcmanus]

Do you know where I would find this info?  These are the chemicals I am dealing with: nitric, hydroflouric, hydrochloric, sulfuric, and (non-acids) potassium hydroxide and soldium hydroxide.
Thanks

[Borek]

Do you know where I would find this info?  These are the chemicals I am dealing with: nitric, hydroflouric, hydrochloric, sulfuric, and (non-acids) potassium hydroxide and soldium hydroxide.

Sulfuric and nitric are miscible with water, so they can have 100% concentrations. As for the rest - look for CRC handbook.

And remember that activities will very strongly influence pH values for concentrated solutions. I am not aware of ANY method of calculating pH in such situations (it can be measured though).

[smcmanus]

If a chemical says that it is concentrated, that means that there is a high concentration of that chemical there.  But how would you figure what % concentration it is?
Thanks

[Blueshawk]

If bottled, usually has molar concentration, ie in chemical store room,  if store bought, as in dishsoap,  most have a % active ingredient on the label somewhere.  That will let you know percent volume of container for that particular ingredient..  Then with chemical formula, and maybe some other data, you can find molar concentration.

[smcmanus]

I really know nothing about chemistry.  I am doing a theoretical paper on material resistance vs. harsh chemicals.  I looked in chemical resistance guides and some list a 'concentrated' chemical being okay at a certain temperature.  For my paper I need to display this as a %.  It looks as though hydrochloric is the only chemical listed as concentrated or dilute.  Could I assume concentrated is say 100% and dilute is 1-2%?
Thanks

[woelen]

The concept of pH only is defined at moderate concentrations. For highly concentrated acids, like 50% HNO3 or almost pure H2SO4 the concept of pH at best has a doubtful meaning. This is because the Ka values for acids assume that the concentration of the acid and its conjugated base is low, compared to the concentration of water, in which it is dissolves. This assumption is valid when the concentration of the acid does not exceed a few percent, but for higher concentrations the error becomes progressively higher and at almost 100% concentration the concept of pH is useless.

[Borek]

This is because the Ka values for acids assume that the concentration of the acid and its conjugated base is low, compared to the concentration of water, in which it is dissolves.

No. When you use activities instead of concentrations pH has a sense regardless of the acid concentration. The problem, is, there is no theory strong enough to make such calculations for high ionic strengths, so when the concentrations go higher it is better to do measurements instead of calculations.

[Blueshawk]

Could I assume concentrated is say 100% and dilute is 1-2%?
Thanks

NO,  100 % concentrated HCl is dangerous, if it van be that concentrated I forgot for that one, and some acids cannot even have 100% concentrate.  I think H2SO4 can be 100%, but it would be very strong acid, like a negative pH value for acidity.

To be safe for a paper, i would say concentrated could be from 30% to 60%, depending on the solution.

The stronger the acid the less you need, so the lower the % concentrate.

If anyone else has suggestions on a mean % concentrate in general, correct me If I am wrong , thx.