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Chemistry Forums for Students => High School Chemistry Forum => Topic started by: fudawala on June 26, 2008, 04:04:38 PM

Title: Calculating the number of particles per milliliter of CdSe
Post by: fudawala on June 26, 2008, 04:04:38 PM
Hi,

I have a question that troubles me. I want to calculate the number of particles per milliliter of Cadmium Selenide? The thing is that I don't know where to start? Can I someone help me please.

Thanks
Title: Re: Calculating the number of particles per milliliter of CdSe
Post by: Borek on June 26, 2008, 05:09:28 PM
Will you be able to calculate number of molecules knowing mass of the substance?
Title: Re: Calculating the number of particles per milliliter of CdSe
Post by: fudawala on June 26, 2008, 09:17:44 PM
Hi,

The molar mass of the substance is 191.37 grams per mole. How do I get the number of molecules from this mass?

Thanks
Title: Re: Calculating the number of particles per milliliter of CdSe
Post by: nj_bartel on June 26, 2008, 09:20:33 PM
(MassSample) / (Molar Mass)
Title: Re: Calculating the number of particles per milliliter of CdSe
Post by: fudawala on June 26, 2008, 09:25:06 PM
What do you mean about the mass of the sample? Is this the right way to calculate the number of molecules per millimeter? Thanks.
Title: Re: Calculating the number of particles per milliliter of CdSe
Post by: Borek on June 27, 2008, 03:34:58 AM
If you know number of moles, can you calculate number of molecules?

If you know mass sample and molar mass of the substance, can you calculate number of moles?
Title: Re: Calculating the number of particles per milliliter of CdSe
Post by: fudawala on June 27, 2008, 09:38:49 AM
Yes you can. So the mass of the sample is 50 milligrams and the molar mass is 191.37 grams per mole. Now if I divide the mass sample over the molar mass I should get the number of moles right. If I multiply the number of moles with avogrado's number I should get the number of molecules per milliliter right.
Title: Re: Calculating the number of particles per milliliter of CdSe
Post by: Borek on June 27, 2008, 11:06:19 AM
So the mass of the sample is 50 milligrams and the molar mass is 191.37 grams per mole. Now if I divide the mass sample over the molar mass I should get the number of moles right. If I multiply the number of moles with avogrado's number I should get the number of molecules per milliliter right.

Mostly OK, but 50 mg is not mass of one milliliter of the CdSe. To calculate that you need one more information, about the solid density.
Title: Re: Calculating the number of particles per milliliter of CdSe
Post by: fudawala on June 27, 2008, 11:58:58 AM
Well the solid density is about 5.816 g per cubic centimeter.
Title: Re: Calculating the number of particles per milliliter of CdSe
Post by: Borek on June 27, 2008, 12:35:09 PM
And you still don't know what to do next?
Title: Re: Calculating the number of particles per milliliter of CdSe
Post by: fudawala on June 27, 2008, 02:15:46 PM
No, I don't know what to do next. Can you explain to me please. Thank You.
Title: Re: Calculating the number of particles per milliliter of CdSe
Post by: fudawala on June 28, 2008, 07:20:12 PM
Hi Borek,

The solid density is about 5.816 grams per cubic centimeter. Do I convert the 50 milligrams to grams and the cubic centimeter to one milliliter?

What I know so far is that: 1 milligram = .001 grams, and 1 milliliter = 1 cubic centimeter.
Title: Re: Calculating the number of particles per milliliter of CdSe
Post by: Borek on June 29, 2008, 03:54:52 AM
You are asked to calculate number of molecules in 1 mL. What is mass of the substance in 1 mL? Number of moles in this mass? Number of molecules in this number of moles?
Title: Re: Calculating the number of particles per milliliter of CdSe
Post by: fudawala on July 01, 2008, 01:59:22 PM
Hi Borek,

How do you claculate the mass of a substance of one milliliter?

Thanks
Title: Re: Calculating the number of particles per milliliter of CdSe
Post by: sjb on July 01, 2008, 02:42:10 PM
Hi Borek,

How do you claculate the mass of a substance of one milliliter?

Thanks

I'm not Borek, but do your previous insights below help?

Hi Borek,

The solid density is about 5.816 grams per cubic centimeter. Do I convert the 50 milligrams to grams and the cubic centimeter to one milliliter?

What I know so far is that: 1 milligram = .001 grams, and 1 milliliter = 1 cubic centimeter.
and
Well the solid density is about 5.816 g per cubic centimeter.
Title: Re: Calculating the number of particles per milliliter of CdSe
Post by: fudawala on July 01, 2008, 03:01:48 PM
Hi sjb,

The concentration of the sample is 10 mg/mL.
Title: Re: Calculating the number of particles per milliliter of CdSe
Post by: enahs on July 01, 2008, 03:05:39 PM
Outline:

You use the density to convert volume to mass.
You use the mass and Molecular Weight of the compound to convert to # of moles.
You use the # of moles and Avagrado's Number/Constant to convert to # of molecules/particles.

You now have the # of molecules/particles in that volume.

Do the math. Follow the units and cancel them out.
Title: Re: Calculating the number of particles per milliliter of CdSe
Post by: fudawala on July 01, 2008, 03:16:48 PM
The only problem that I have is to convert the volume to mass using the density. Do you know how to do that. Thanks.
Title: Re: Calculating the number of particles per milliliter of CdSe
Post by: enahs on July 01, 2008, 03:28:48 PM
Use your units.

This is not the numbers for your problem, just some made up.


Density: mass / volume (example, g/mL)

So, if I have volume * density I am left with mass. Or mass / density I am left with volume.

:
density of 33.3g/mL

55 mL * 33.3 g =    1,831.5 g
              1 mL

density of 10g/mL

6 g * 1 mL    = 0.6 mL
         10 g

         
Title: Re: Calculating the number of particles per milliliter of CdSe
Post by: fudawala on July 01, 2008, 03:35:31 PM
Can I ask you a question. On the sample it says the concentration which is 10 mg per mL. It also labels that the nanomole/mL is 69.4. I just want to know which one is the mass of the substance of 1 mL. Thanks again.
Title: Re: Calculating the number of particles per milliliter of CdSe
Post by: enahs on July 01, 2008, 03:46:07 PM
10mg/mL implies there are 10 mg of CdSe per mL of solvent.

Where as 69.4 n moles/mL implies there are 13.28 μG (micro grams) per mL of solvent.

Those contradict each other.


Perhaps you should start over fresh. What is the question (word for word) and what are you given.
Title: Re: Calculating the number of particles per milliliter of CdSe
Post by: fudawala on July 01, 2008, 05:19:07 PM
My question is to calculate the number of molecules per milliliter of a solution?

The solution is a Core Shell Evidot in Toluene of a Hops Yellow color. The molecular weight is given to me which is 20 micrograms/nmol. This value is given to me from the table.

Another thing that is given to me is listed on the solution bottle itself. It labels the absorption and emission peaks in wavelength. The absorption peak is 565 nanometers. The emission peak is 578 nanometers.

The concentration is given to me which is 10 mg/mL.
The nmol/mL is also given to me which is 69.4.

I'm being asked to find the number of particles per milliliter of this substance? I don't know how to solve the problem from this given to me?

Thanks Again. I appreciate your help. Please I need your help on this one.
Title: Re: Calculating the number of particles per milliliter of CdSe
Post by: Borek on July 01, 2008, 05:30:57 PM
Either you are given wrong information, or you are misreading it. 20 μg/nmol is at least expressed in strange units, but it means molar mass of 2 x 105 g/mol. Then you are given information that in each mL there is 10 mg of substance and it is 69.4 nmol. This in turn means molar mass in the range of 1.4 x 105. Molar mass of CdSe is 191.37 g/mol. As nothing fits, I doubt anyone will be able to help.
Title: Re: Calculating the number of particles per milliliter of CdSe
Post by: fudawala on July 01, 2008, 06:22:14 PM
Can you tell me how did you get 2 X 10^5 of one molar mass. The other molar mass of 1.4 X 10^5.

The molecular weight is not 20 micrograms per nanomole. Gave wrong info. But the molecular weight is 23 micrograms per nanomole.
Title: Re: Calculating the number of particles per milliliter of CdSe
Post by: Borek on July 01, 2008, 06:25:36 PM
20 x 10-6/10-9 & 0.01/69.4 x 10-9.

And sorry, typo, first one should be 20k.

Molar mass is mass of the sample divided by the number of moles in the sample.
Title: Re: Calculating the number of particles per milliliter of CdSe
Post by: fudawala on July 01, 2008, 06:35:13 PM
.01 is the density right because the concentration was 10 mg/mL.
Title: Re: Calculating the number of particles per milliliter of CdSe
Post by: fudawala on July 01, 2008, 06:57:36 PM
Can someone explain me what is a molar extinction coefficient at first exciton up to 10^5. The Core Shell Evidot of the Hops Yellow color is about 0.98 (molar extinction coefficient at first exciton [10^5]).
Title: Re: Calculating the number of particles per milliliter of CdSe
Post by: fudawala on July 01, 2008, 07:08:59 PM
Hi enahs,

I just want to know how did you get 13.28 micrograms from 69.4 nanomoles.

Thanks
Title: Re: Calculating the number of particles per milliliter of CdSe
Post by: enahs on July 01, 2008, 07:32:54 PM
You said the compound was Cadmium Selenide, which as a molecular weight of 191.4 g/mol.
You said it says 69.4 nanomoles / 1mL

69.4 nanomoles = 69.4 (10-9)moles

69.4 (10-9) moles * 191.4 g  = 13.2 (10-6) g = 13.2 μg
                           1 mole


The molar extinction coefficient is the older term for the molar absorptivity coefficient.  It is a constant for a specific substance in a specific solvent. It relates the absorbance of the compound to its concentration and path length in the Beer-Lambert law.


Your material is not juse Cadmium Selenide though as you previously said. Based on the "Core Shell Evidot" comment, it is actually a more complicated mixture with Zinc Sulfide and such, not just the CdSe. But who knows what proportions the ZnS and CdSe are in, so the molecular weight can not be added up from constitute parts.


The best you can do is take the bottles word.

You say the bottle says 69.4 nmol/mL and 10mg/mL, which means there are 10mg/69.4 nmol
10 (10-3) g     *         1 mL          = 144,092 g/mol
  1 mL                   69.4 (10-9)mol

But your quantum dots also probably have amines in them as well to "cap" them.

But back to your first question:
Quote
My question is to calculate the number of molecules per milliliter of a solution?
You are told there is 69.4 nmol's of stuff in 1 mL. Therefor, multiplying it by avagrados number gives you how many molecules of stuff in 1mL.

This is a very poor question and answer though.
Title: Re: Calculating the number of particles per milliliter of CdSe
Post by: fudawala on July 01, 2008, 07:54:58 PM
Can I ask a question about the mixture between CdSe and Zinc sulfide. You said that when you mix these two compounds the molecular weight does not add. Can you tell me why?
Title: Re: Calculating the number of particles per milliliter of CdSe
Post by: enahs on July 01, 2008, 08:02:08 PM
Because you do not know the proportions the two are mixed. They are not forming a new compound with a chemical reaction in this case, you have the ZnS as a physical shell around the CdSe.
There could be 3 ZnS for every one CdSe, or 500. Plus there is most likely an amine in there as well, again you do not know the proportion.

Title: Re: Calculating the number of particles per milliliter of CdSe
Post by: fudawala on July 01, 2008, 08:05:33 PM
Now I understand, hey thanks a lot.

But, how would I solve for the number of particles per millimeter if the molecular weight is no use. Because you said that the ZnS and the CdSe are inter mixed with each other.
Title: Re: Calculating the number of particles per milliliter of CdSe
Post by: enahs on July 01, 2008, 08:10:13 PM
You can only trust what was given to you as 69 nano moles of stuff per 1 ml.

If you have 1 ml of the stuff, and 69 nano moles in it, you then have 69 nano moles * avagrados number of molecules

Avagrados # is
# of "stuff"
  1 mol

And in this case, the "stuff" is molecules. Avagrados number is just like a dozen, just a number. You can have a dozen eggs or a dozen waffles.

But remember Avagrados number is in mols, so you must convert nano mols to mols first.

Title: Re: Calculating the number of particles per milliliter of CdSe
Post by: fudawala on July 01, 2008, 08:19:26 PM
So Once I know Avogrado's Number which is 1 mole = 6.02 X 10^23 atoms right and when I convert 1 nanomole to moles which is:

1 nanomole = 10^-9 mole right. This will tell me the number of particles per milliliter right. Thanks again.
Title: Re: Calculating the number of particles per milliliter of CdSe
Post by: fudawala on July 01, 2008, 08:23:50 PM
Can I ask you what happens to the density of these two compunds. Should I consider it in his problem?
Title: Re: Calculating the number of particles per milliliter of CdSe
Post by: Hapto on July 25, 2008, 02:21:27 PM
Are you sure that you read the label correctly?  Quantum dots have both the wavelength of absorption (useful for determination of size of the dots) and the weight percent of the suspended particles (useful for concentration of the dots) on the label.

Are you sure that the label didn't say 694 nm (nanometers! ... not nanomolar) and 10 mg/ml?