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Topic: Why doesn't density change as concentration changes...?  (Read 5306 times)

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Offline Lo.Lee.Ta.

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Why doesn't density change as concentration changes...?
« on: September 15, 2013, 02:17:55 PM »
I was just thinking about this when calculating the limiting reagents...

Concentration (M) is about how compact particles are in a solution. M = mol/L

Density = g/L  ...So it also seems like density also has to do with the compactness of particles... But they say the only thing that changes density is temperature and pressure...

My example is at STP:
It seems like if I had a very dilute solution of HNO3, the molecules would be very spread out.
So if I had 1mL of this dilute HNO3 solution and tried to find the number grams, it SEEMS that the number of grams would be small.

BUT, I should be able to figure this out from the density also, right?

1mL HNO3 + (1.5129g / 1mL) = 1.5129g of HNO3 in 1mL of solution.

If I had a very concentrated solution of HNO3, the molecules would be very compact.
So if I had 1mL of this concentrated solution, it SEEMS like I would have many grams of HNO3.

But if I tried to use the density of HNO3 as the conversion factor, the grams of the concentrated solution would be the SAME as the dilute solution!  ???

...This doesn't make any sense...
If the above is not right (which is likely #=_=), is there a way to know the grams from only the mL of solution?


So confused now.
Thank you SO much for the assistance! :)
 

P.S. By the way, I changed "help" to "assistance." Why does it substitute "*delete me*"  for the word "help" at the end here on chemical forums??
That's irritating.  >:(
« Last Edit: September 15, 2013, 05:51:12 PM by Lo.Lee.Ta. »

Offline Nobby

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Re: Why doesn't density change as concentration changes...?
« Reply #1 on: September 15, 2013, 02:48:56 PM »
You forget the Percentage.

V1 x P%1 x Rho1 = V2 x P%2 x Rho2


Offline Borek

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Re: Why doesn't density change as concentration changes...?
« Reply #2 on: September 15, 2013, 02:56:44 PM »
M = mol/L

Moles of what, volume of what?

Quote
Density = g/L

Grams of what, volume of what?
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Offline Lo.Lee.Ta.

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Re: Why doesn't density change as concentration changes...?
« Reply #3 on: September 15, 2013, 05:18:34 PM »
@Borek:

M = mol solute/ L solution

Okay, looked density up:

D = g solution/ mL solution

...Oh, well, it looks like the density is no longer useful to me for converting from mL to g! :/
What's the point of knowing that 2.7mL of methyl benzoate with a density of 1.0837g/mL has a mass of 2.9g?
The mass means: 2.9g = solute mass + solvent mass, right? :/

Hm. So, there is no way to know the solute mass from only the density and mL of solution, right?
 :(

...Well, how then am I supposed to calculate the limiting reagent from lab...?
They only told us we have 2.7mL methyl benzoate, 3.0mL concentrated nitric acid, 3.1mL concentrated sulfuric acid, etc.

"Concentrated" does not tell me the exact molarity to figure out the grams of solute! #=_=

Help please?  ???
Thanks!



Edit: The ONLY thing different I notice when reading through the lab again is this sentence: "Using a glass pipet, slowly add 3.0g of methyl benzoate (2.768mL) to the cooled sulfuric acid solution."

I actually used 2.7mL of the methyl benzoate solution.

Does the 3.0g mean that there are 3g of methyl benzoate solute per 2.768mL solution, OR does it mean that 2.768mL of solution has a weight of 3g (3g = mass of methyl benzoate + mass of solvent)???


If the first was correct, I could say:

(3g / 2.768mL) = (x / 2.7mL)
x = 2.93g of methyl benzoate solute


If the latter was correct, I still couldn't figure out the grams of methyl benzoate solute...


« Last Edit: September 15, 2013, 05:47:38 PM by Lo.Lee.Ta. »

Offline Borek

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Re: Why doesn't density change as concentration changes...?
« Reply #4 on: September 15, 2013, 06:09:37 PM »
So, there is no way to know the solute mass from only the density and mL of solution, right?

If you know only density and volume - that's not enough information.

Quote
They only told us we have 2.7mL methyl benzoate, 3.0mL concentrated nitric acid, 3.1mL concentrated sulfuric acid, etc.

"Concentrated" does not tell me the exact molarity to figure out the grams of solute! #=_=

Concentrated nitric acid means around 68% (that's a typical stock solution), concentrated sulfuric acid means around 98% (that's a typical stock solution).

Quote
Does the 3.0g mean that there are 3g of methyl benzoate solute per 2.768mL solution, OR does it mean that 2.768mL of solution has a weight of 3g (3g = mass of methyl benzoate + mass of solvent)???

This is not a SOLUTION, this is a pure substance. You can use directly the definition of density to calculate its amount.
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Offline Lo.Lee.Ta.

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Re: Why doesn't density change as concentration changes...?
« Reply #5 on: September 15, 2013, 08:42:18 PM »
Okay, so since methyl benzoate is pure (not a solution), then we can do dimensional analysis from the 3g, right?

3g methyl benzoate x (1 mol / 136.15g) x (1 mol methyl-m-nitrobenzoate / 1 mol methyl benzoate) x (181.15g / 1 mol)

= 3.9g methyl-m-nitrobenzoate

I don't even know if I need to find the limiting reagent, but we COULD try to find it here by using the percentages of H2SO4 and HNO3, right?

3.0mL HNO3 * 68% = 2.04mL pure nitric acid

3.1mL H2SO4 * 98% = 3.038mL pure sulfuric acid

Since the substances we have are pure (not solutions), couldn't we use density like this:

1.84g/mL = (mass / 3.038mL)
mass of H2SO4 = 5.59g

1.5129g/mL = (mass / 2.04mL)
mass of HNO3 = 3.09g

Molecular Formula:
 methyl benzoate + NO2 + HSO4 + H2O  :rarrow: methyl-m-nitrobenzoate + H2SO4 + H2O

5.59g H2SO4 x (1 mol / 98.079g) x (1 mol NO2 / 1 mol H2SO4) x (46.0055g / 1 mol NO2)
= 2.62g NO2

3.09g HNO3 x (1 mol / 63.01g) x (1 mol NO2 / 1 mol HNO3) x (46.0055g / 1 mol NO2)
= 2.26g NO2

So HNO3 would be the limiting reagent for the electrophile generation.


2.26g NO2 x (1 mol NO2 / 46.0055g) x (1 mol methyl-m-nitrobenzoate / 1 mol NO2) x (181.15g / 1 mol)
= 8.90g methyl-m-nitrobenzoate

So methyl benzoate would definitely be the limiting reagent.

So is this the right way to do things?
Thanks so much!!! :)


Offline Borek

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Re: Why doesn't density change as concentration changes...?
« Reply #6 on: September 16, 2013, 03:10:36 AM »
I don't even know if I need to find the limiting reagent, but we COULD try to find it here by using the percentages of H2SO4 and HNO3, right?

3.0mL HNO3 * 68% = 2.04mL pure nitric acid

3.1mL H2SO4 * 98% = 3.038mL pure sulfuric acid

Since the substances we have are pure (not solutions), couldn't we use density like this:

1.84g/mL = (mass / 3.038mL)
mass of H2SO4 = 5.59g

1.5129g/mL = (mass / 2.04mL)
mass of HNO3 = 3.09g

That's in general not a correct approach to this type of problem, as the percentage I suggested was a mass percentage, and the density you found (somewhere) was a density of a pure substance - and while you used concentrated acids, they were most likely solutions of concentrations I suggested earlier. So you should find density of the solution of a given concentration (that would be around 1.40 g/mL for 68% nitric acid, and 1.84 g/mL for a 98% sulfuric acid), use these densities to find mass of the acid solution, then use percentage concentration to calculate mass of the acid present in the solution - and from there follow with the stoichiometry.

Your result - that benzoate is the limiting reagent - is most likely correct, that's how we usually do, we use excess of acids for nitration to shift the equilibrium to the right. Acids are the cheapest reagents used here.
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