How do i prepare this stock solution plz ?

[WESAM]

Hi guys ,

I would be very thankful if is you could help me in preparing this , as i have been trying to find in the internet but with no success :

1- How would you prepare 1 litre of solution of the following concentrations

a- 1 Molar
b- 1% ( w/v) of the cation
for the following salt ?

Mg(NO3)2 6H2O purity 96.2% Mwt 256.41

note: don't forget the impurity and 1% it is for the cation .



Thanks in advance

[tom jupille]

Don't take this the wrong way, but that's something you should find in your high-school chemistry text. That said, here are a couple of hints to get you started:

a- 1 Molar

What is the definition of molarity?
If the compound were pure, how many grams would make up one mole?
Given that the purity is only 96.2%, how many grams would you have to weigh out to make up one mole (remember, it will take *more*).

b- 1% ( w/v) of the cation

How many grams of the cation do you need?
What is the cation in this salt?
What is it's atomic weight?
What percentage of the total molecular weight is represented by the cation?
If the compound were pure, how many grams of the compound would you have to weigh out to get the amount of cation required?
Given that the purity is only 96.2%, how much more you you have to weigh out?
Since you only specified concentration to 1 significant figure, it's safe to assume that the solvent is water and that the density of the final solution is still about 1 kg/L.

[WESAM]

Tom , thanks but believe me or not this is question is an exam for Master student in chemistry in UK and no one answer it correctly .So it comes now into my project and i ask many people ( ues they now how to prepare it as Molar in 100 % purity but less than this how can i include it ) .If you can recommend me any text book that can explain this i would very gratefull .

Any way , saying in secondery school tex book that's penetrate my heart with an arrow , but don't worry i deserve it as i am weak in calculation

Thanks

[tom jupille]

Okay,

1 Molar = 1 mole / liter of solution. 1 mole of your compound would be 256 grams (if it were pure). Since it's impure, you will need a bit more, so divide the 256 grams by the purity (0.96) = 267 grams. Since you only specified to 1 significant figure (1 molar, not 1.0 molar), you can ignore density effects and round the weight to 270 grams. So: weigh out 270 grams of your compound, transfer to a 1 L volumetric flask, fill to the mark with water, and Robert is your mother's brother.

Similar logic goes for the 1% Mg++. 1% is 10g/L. Magnesium has an atomic weight of 24, so it makes up (24/267 = ) 9% of your total compound. So, to get 10g of Mg, you would have to dissolve (10/0.09 = ) 111 grams of pure compound. Remember, it's impure, so you will need a bit more: (111/0.96 = ) 116 grams. Remember, you only asked to 1 significant figure, so weigh out 120 grams of your compound, transfer to a 1 L volumetric, and fill to the mark with water.

[WESAM]

Thanks Tom for your help

i am sure your answer is correct but why the supervisor said to me ( he gave me only the final answer as the following )

1M= 266.5385
1% = 109.6415

[tom jupille]

Ask your supervisor (diplomatically) to explain his calculations to you.

When he does, inquire (respectfully, of course) how many significant figures he expects. 1 M = 270g; 266.5385g implies 1.00000 M

Re the 1%, 111 grams would be the correct answer if the compound were pure -- but it's not!

[Peter Apps]

And now I'm wondering how many of the students that got it wrong actually got it right

Peter

[WESAM]

Tom thanks i know what he did

Mwt= 256.41 /0.962= 266.5385

But why he didn't round it to ( 270 g ) . Honestly , i feel Tom method is very precise

Thanks



My dear Peter , i didn't understand what you said but we are six student from Zology department are studying master in chemistry which is very hard to us to come to this new field .( i am really surprise why they are talking about Atom and they didn't see it , how logical this )

[Peter Apps]

Hi Wesam

I think that we can agree that Tom's answers are correct. It can be argued that the supervisors answer to question 1 is also correct, though unnecessarily precise. Nonethless, the supervisor's answer to question 2 is wrong, because it does not take the (im)purity into account. So I was wondering if any of your fellow students gave the answer of 116 g for question 2 (which is correct) but were marked as having the wrong answer.

As a zoologist myself I have to comment that, since this level of question is high school, not university level chemistry, your being zoologists is not an escape clause

Mind you I have known chemists with B.Sc.s who could not calculate the dilutions needed to make up a calibration series

Peter

[tom jupille]

After re-reading my original reply, I realize that I goofed on the second part! In effect, I applied the impurity correction twice.

It should have read:
Magnesium has an atomic weight of 24, so it makes up (24/256 = ) 9.4% of your total compound. So, to get 10g of Mg, you would have to dissolve (10/0.094 = ) 106 grams of pure compound. Remember, it's impure, so you will need a bit more: (106/0.96 = ) 111 grams. You only asked to 1 significant figure, so weigh out 110 grams of your compound, transfer to a 1 L volumetric, and fill to the mark with water.

[WESAM]

Hi Wesam

As a zoologist myself I have to comment that, since this level of question is high school, not university level chemistry, your being zoologists is not an escape clause

Mind you I have known chemists with B.Sc.s who could not calculate the dilutions needed to make up a calibration series

Peter

I think this * is not related to my question* ! , and whether or not you are zoologist, ( i am not an animal to repeat and repeat something can not help because i can understand this when Tom gently posted ! ).

Please try to be more nice in your writing and we know you are very genious .

You are Fired ( your boss )

Thanks

[Peter Apps]

WESAM

It was you, not me, who brought up your being zoologists as if that was an explanation of why you all got the wrong answer at M.Sc. level to a high school chemistry question:

" but we are six student from Zology department are studying master in chemistry which is very hard to us to come to this new field .( i am really surprise why they are talking about Atom and they didn't see it , how logical this )"

Interdisciplinary collaborations can be hugely productive - but for them to work properly the various specialists have to have at least a basic common language - which means that they have to understand at least the basics of each other's science. Hence the need for zoologists to know what a Mole is, how atoms combine to make compounds, etc etc etc.

Being able to write intelligibly is useful for everyone of course.

Peter

[KrypticKhemist]

I know this is off topic but I see that the post is now almost a month old so feel better in asking- What interest would a zoologist have in chemistry? I understand how chemistry is relevant to all sciences believe me. But why would a zoologist get their BS in biology/zoology and then take a masters in Chemistry? To me that seems like it would be very difficult to jump into graduate level chemistry courses when organic chem (maybe) was your highest previous chem course.

Maybe I misunderstood? Is it a Masters in Zoology but just includes this chem course?

I'm interested.

[Peter Apps]

Hi Kryptic

Good question ! I can't answer on behalf of Wesam and his colleagues, but in my case my interest in chemistry stems from a need to indentify the chemicals that mammals (specifically African wild dogs) use to send social messages. This involves mainly GC-MS, and I unashamedly base nearly all my identifications on library searches (which is what proper chemists do as well) supported by retentions, expectations based on metabolism and what has been found in other mammals, and a firm conviction that a library search that throws up something with six deuteriums in it is not likely to reflect what was actually in the sample.

If I run into a real unknown I will pass it on to real chemists who have the skills, and the hardware, to do high res MS, NMR, infra-red, wet chemistry etc. I would almost say that the most useful piece of chemistry that I know is knowing that the boundaries lie way beyond what I can do, and knowing what the paths are to get there, who to ask, and what to ask them in terms that they can understand - in other words I have to talk chemistry to chemists.

Of course, there are basic "chemical" operations involved in the analyses that I do - molecular weights, vapour pressures, pH, partition, issues of analyte stability, dirty inlets and deteriorated columns, and the basic arithmatic of calculating dilutions, are just as important in my lab as they are anywhere else, and I have to know how to handle them.

Research into mammal chemical communication is almost a paradigm for how poor collaboration between different disciplines can slow things down. Teams of really good natural products chemists produce lists of the chemicals definitively identified in various secretions and excretions - with no link to biology. Biologists happily run series of samples extracted with the wrong solvent, run on the wrong column and analysed with the wrong statistics, and get no further than a demonstration that different classes of animal (say males vs females) have different patterns of volatiles. The few success stories are written by teams that include both good chemists and good biologists, who know how to talk to one another. There is probably a doctorate in the sociology of science here somewhere !

Collaborations cannot work as well as they should if the different specialists look on each others skills as impenetrable dark arts based on esoteric knowledge. So a zoologist for whom Mole or vapor pressure are profound mysteries has to leave all of the chemistry to the chemists, and the chemists will be unable to understand why one particular individual mouse, which looks like all the other mice and has been bred over 200 generations to be genetically indistinguishable to all the other mice, has to be compared with another particular one.

Molecular biology has brought biology and chemistry (and a fair chunk of physics) closer than they have ever been, so there is more need than before for the two disciplines to be talking the same language. Nobody needs to be an expert in all the fields, but the basic tool kit, which has reading, writing, arithmatic and the basic concepts of science in its top drawer has to be available to everyone. Whether re-jigging a zoologist into a chemist at M.Sc level is the way to do it it questionable - I would prefer to see people going into their undergraduate courses with those tools already in their pockets.

Whoops, ran on a bit there !

Peter

[WESAM]

Peter

you've done well in explaining the issue

Thanks