Sciencemadness Discussion Board

Concentration of hardware store acids

francis - 20-7-2011 at 14:37

Hi,

I have Diggers brand hydrochloric acid, labelled as containing 280g HCl / L

I am trying to work out the wt% (w/v) and the molarity.

My textbook (Harris, 'Quantitative Chemical Analysis') lists the approximate density as 1.19g/mL

I determined the weight% to be:
(280g/1000g) / 1.19g/mL * 100% = 23.5 wt %

And the molarity to be 6.46 mol/L

Is this correct? Or have I made an error?

----

Then I have 85 wt% food grade phosphoric acid.

I determined the molarity by this method:

(0.85g/mL) * (1.69g/mL density from Harris) = 1.437g/mL * 1000 = 1437g/L

(1437g/L) / 98g/mol FW H3PO4 = 14.67 M

------

Then I have Mo-flo brand Sulfuric acid which lists the contents as 1835g/L

Harris lists the density as 1.84g/mL. I can't work out the percentage or the molarity - both numbers I get are off...

Any help is much appreciated, thanks alot.

smaerd - 20-7-2011 at 15:48

In my experience, never trust and MSDS, or a label on a consumer product when it comes to purity or concentration. The best way to determine molarity from such a product would be, experimentally. A basic titration could resolve these questions instead of guessing. If accuracy is critical then I would do this for each time you buy the product. Keep in mind also that, there are likely other impurities in these products as well.

Magpie - 20-7-2011 at 15:57

It's best to learn the definition of molarity and work from there.

For the HCl you have 280g/L, and by definition molarity is the moles/L, so this one is easy:

M = (280g/L)/(36.46g/mole) = 7.68 moles/L = 7.68M

For the phosphoric acid you set it up a little strange IMO but got the right answer:

M = (0.85)(1690g/L)/(98g/mole) = 14.66 moles/L = 14.66M

For the sulfuric:

You will have to go to a table of wt% vs density, or molarity vs density. If the latter you will have your answer directly; if the former you calculate it as done above. For example, if the wt% is 96 then:

M = (0.96)(1835g/L)/(98.08g/mole) = 17.96 moles/L = 17.96M

francis - 20-7-2011 at 21:51

Hi, thanks very much for the responses.

I intend to standardise these solutions smaerd...but I'd like an approximate knowledge of their molarity.

Magpie:

Why didn't you factor in the density for the HCl calculation but you did for the H2SO4 calculation and the H3PO4 calculation?

Magpie - 21-7-2011 at 09:22

Quote: Originally posted by francis  

Magpie:

Why didn't you factor in the density for the HCl calculation but you did for the H2SO4 calculation and the H3PO4 calculation?


For the HCl it was given that the HCl content was 280g/L. That is all that is needed. To find the g/L of H3PO4 and H2SO4 the density was required. You have to take a close look at the definition of molarity and work from there.

francis - 21-7-2011 at 21:33

Quote: Originally posted by Magpie  
Quote: Originally posted by francis  

Magpie:

Why didn't you factor in the density for the HCl calculation but you did for the H2SO4 calculation and the H3PO4 calculation?


For the HCl it was given that the HCl content was 280g/L. That is all that is needed. To find the g/L of H3PO4 and H2SO4 the density was required.


But the H2SO4 content was also given as 1835g/L?

Why do you multiply by the density for H2SO4 but not HCl?

Magpie - 21-7-2011 at 21:48

Quote: Originally posted by francis  

But the H2SO4 content was also given as 1835g/L?


No, just the density was given. This means a liter weighs 1835g. You don't know what fraction of that is H2SO4 and what fraction is just water.
Quote: Originally posted by francis  

Why do you multiply by the density for H2SO4 but not HCl?


I assumed a wt% of 96% just to show an example calculation. In this case the weight of a liter is needed, which you get from the density, ie, wt/L = (density)(1000g/L).

Don't look for a "one size fits all" formula. Start from the definition of molarity and use what data is necessary to make the computation.

francis - 21-7-2011 at 22:00

Ahhh, yes, sorry Magpie! I have been inaccurate.

The H2SO4 bottle actually says, "1835g/L Sulfuric Acid"

The HCl bottle says, "280g/L Hydrochloric acid".

So in the case of the sulfuric acid, its just the same calculation you used for the hydrochloric, ie 1835g/L / 98 g/mol = 18.72 mol/L

Would that be right?

Thanks alot for your help, and especially with your comment on density, I belive I understand this now.

Magpie - 22-7-2011 at 07:25

Quote: Originally posted by francis  

The H2SO4 bottle actually says, "1835g/L Sulfuric Acid"

The HCl bottle says, "280g/L Hydrochloric acid".

So in the case of the sulfuric acid, its just the same calculation you used for the hydrochloric, ie 1835g/L / 98 g/mol = 18.72 mol/L

Would that be right?


With your sulfuric acid it's somewhat ambiguous. I'm guessing that the 1835g/L includes the water content as con sulfuric is usually almost all acid, ie >94%, or so.

But with the HCl it is clear that the 280g/L represents only the HCl and does not include the water in which it is dissolved. No liter of an aqueous solution is going to have a density as low as 0.280.

So, to answer your question, I'd say that your calculation is not strictly correct. You need to incorporate the wt% H2SO4 in the calculation just as I showed in my sample calculation above.

I see how this can be confusing. But it is important to get it right as you will put it to practical use throughout your chemistry work and hobby.