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Offline JZ_1

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Mass spectra and Hydrates
« on: August 13, 2005, 05:38:40 PM »
Here's problem number 1:
--Statement--
• Hydrogen and chlorine both have two naturally occurring isotopes:
 1H (1.0078 amu; abundance = 99.985 percent) and
 2H (2.01410 amu; 0.015 percent abundance),
 35Cl (34.969 amu;  75.53 percent abundance) and
 37Cl (36.966 amu; 24.47 percent abundance).

--Question--
(a) How many peaks would be expected in a mass spectrum of HCl(g)?
(b) At what molecular weights will peaks be observed?
(c) What should be the relative intensities of the peaks?
--??--

A) Is it 4? (lucky guess?) I read somewhere that Bromine, Br2 has 2 isotopes but with 3 peaks...i think because of 1 isotope joining with another isotope..

OUT OF CURIOSITY..if the question is on H2 ...Will it have 3 peaks as well?

B and C)  i have no idea how to answer or guess what they are...
I STILL DONT KNOW HOW MANY PEAKS ARE PRESENT...
How do u find the relative intensities?

Here's Problem number 2:
• Washing soda, a compound used to prepare hard water for washing laundry, is a hydrate, which means that a certain number of water molecules are included in the solid structure. Its formula can be written as Na2CO3 • xH2O, where x is the number of moles of H2O per mole of Na2CO3.

When a 2.558-g sample of washing soda is heated at 125°C, all the water of   hydration is lost, leaving 0.948 g of Na2CO3. What is the value of x?

...NO idea where to begin..

Well there is 2.558g Na2CO3 • xH2O minus 0.948 g of Na2CO3 = 1.61 g xH2O

...does the 125°C have anything to do with this problem of finding the value of x?...

Also i divide each of these grams by their amu to find their MOLES:
0.948 GRAMS of Na2CO3 = 8.944 X 10-3 MOLES

1.61 GRAMS of xH2O = 0.089345172 MOLES

What do i do next?

AM I suppose to convert them to moles?....i dunno how to begin to find x ...I think i still have some more questions..but these are the ones that concerns me now...

*edited to improve eligibility
« Last Edit: August 13, 2005, 06:22:48 PM by geodome »

Offline Donaldson Tan

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Re:Mass spectra and Hydrates
« Reply #1 on: August 13, 2005, 06:33:55 PM »
Quote
--Question--
(a) How many peaks would be expected in a mass spectrum of HCl(g)?
(b) At what molecular weights will peaks be observed?
(c) What should be the relative intensities of the peaks?
--??--

list out all possible combinations of the H and Cl isotopes.
This should be able to help you (a) and (b).

(c) requires a bit of mathematics - probability. use the probability tree diagram to help calculate the relative intensities of the peaks.

Quote
When a 2.558-g sample of washing soda is heated at 125°C, all the water of  hydration is lost, leaving 0.948 g of Na2CO3. What is the value of x?

Na2CO3.xH2O => Na2CO3 + xH2O
In 2.558g of washing soda, how many moles of water of hydration is present? how many moles of Na2CO3 is present? can you relate the experimental molar ratio to the stoichiometric ratio of products in the above equation?
« Last Edit: August 13, 2005, 06:35:36 PM by geodome »
"Say you're in a [chemical] plant and there's a snake on the floor. What are you going to do? Call a consultant? Get a meeting together to talk about which color is the snake? Employees should do one thing: walk over there and you step on the friggin� snake." - Jean-Pierre Garnier, CEO of Glaxosmithkline, June 2006

Offline JZ_1

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Re:Mass spectra and Hydrates
« Reply #2 on: August 14, 2005, 02:24:44 AM »
list out all possible combinations of the H and Cl isotopes.

How do i kno if the combination is POSSIBLE?

If all H isotopes can combine with all Cl isotopes, then there are 4 Combinations/Possibilities right?
1) 1H -- 35Cl
2) 1H -- 37Cl
3) 2H -- 35Cl
4) 2H -- 37Cl

The molecular weights are just the superscripts added together…right?
So...
1) HCl = 36
2) HCl = 38
3) HCl = 37
4) HCl = 39

I hope i am right...Or do i have to add the amu together?
So...
1) HCl = 35.9768
2) HCl = 37.9738
3) HCl = 36.9831
4) HCl = 38.9801
...Which one is right to find the moler weights?...second one right?

(c) requires a bit of mathematics - probability. use the probability tree diagram to help calculate the relative intensities of the peaks.

PROBABILITY TREE DIAGRAM?…uh…what is it and how does that help?

Shouldn't i do something with the amu or the percent abundance???

HOW do i find the relative intensities of the peaks?

can you relate the experimental molar ratio to the stoichiometric ratio of products?
Thx the x = to 10...thx on that one

o yea...i still need *delete me*!!

Problem number 3:
• Nicotine, a component of tobacco is composed of C, H,
  and N. A 5.250-mg sample of nicotine was combusted,
  producing 14.242 mg of CO2 and 4.083 mg
  of H2O.
What is the empirical formula for nicotine?
If the substance has a molar mass of 160 ± 5 g, what is
  its molecular formula?

I started with .014242 g CO2 and converted to 3.236 X 10-4 MOl of C

Then i did the same thing with H2O, converting it to 4.532 X 10-4 MOL of H.

To find how much O atoms, i'll have to convert 3.236 X 10-4 MOl of C and 4.532 X 10-4 MOL of H to GRAMS and subtract them from 0.005250 g sample of nicotine...
BUT i CAN'T Because there are some O atoms and N atoms in that remainder of grams....HOW do i separate the O and N atoms from the remainder g?


Problem number 4:
• Gallium (GA) consists of two naturally occuring isotopes with masses of 68.926 and 70.926 amu.
a)How many protons and neutrons are in the nucleus of
   each isotopes?
b)The average atomic mass of Ga is 69.72 amu. Calculate
   the abundance of each isotope.

So... i kno that Ga has 31 protons
a) 69Ga  and 71Ga  
    (are the atomic mass of the isotopes correct?)
B) HOW do i calculate the abundance?

    Isn't it something like:
   (68.926amu)*(??? %) + (70.926amu)*(??? %) = 69.72
how do i find out the "???"
...well one of them i can substitute (1.0 - ???%)
…since ??? + ??? equals 1.0

Problem number 5:
• An element X forms an iodide XI3 and a chloride XCl3. The iodide is quantitatively converted to the chlorine when it is heated in a stream of chlorine:
  2XI3 + 3Cl2 => 2XCL3 + 3I2

If 0.5000 g of XI3 is treated, 0.2360 g of XCl3 is obtained.

a) calculate the atomic weight of the element X.
b) Identify the element X.

This problem seems easy..but i dont know how to begin since they gave me GRAMS..and i cant convert it into moles to make a mole to mole comparison... because i dont know what the element X is....

WHAT can i do?..How do i start to solve this problem?

I hope that is enough...5 Problems out of 45 that i have to complete...i wish that is all the problems i have to ask...if i ask any more it'll make me feel dumb....

THX FOR REPLYING!!! THZ FOR YOUR TIME!!! THX FOR YOUR *delete me*!!

Offline Mitch

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Re:Mass spectra and Hydrates
« Reply #3 on: August 14, 2005, 02:33:55 AM »
Deuterium won't be abundant enough for you to be able to see it on a mass spec. And if you could, the peak would be 99% less visible than the proton peak, and would probably be mixed with the background of the intrument.
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Offline JZ_1

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Re:Mass spectra and Hydrates
« Reply #4 on: August 14, 2005, 01:05:59 PM »
Then what ways can i calculate the intensity from the info above..

also, the other questions are a bit hard also...

THX for the *delete me*!

Offline Mitch

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Re:Mass spectra and Hydrates
« Reply #5 on: August 14, 2005, 01:45:28 PM »
You already did, why would I do it again? Just don't worry about the deuterium.
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Offline JZ_1

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Re:Mass spectra and Hydrates
« Reply #6 on: August 14, 2005, 02:00:57 PM »
o..ok..

Forget about problem number 3....that's not really important...i think i got the answer to that....THX

Nicotine 's
empirical formula:  C5H7N
molecular formula: C10H14N2

The should be right....

i'm still working on problem 4 and 5....

Offline sdekivit

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Re:Mass spectra and Hydrates
« Reply #7 on: August 14, 2005, 03:27:58 PM »
probavility tree works as followed:

imagine we have a vase with colored balls. We have in this vase 5 black and 7 white balls. What is the chance we draw 1 balck and 1 white ball with putting back

--> we have 2 draws. The chance to draw a black baal = 5/12 and a white one is 7/12. In the second draw it's the same. We can make a tree from this:


                     black (5/12)
                        |
    black(5/12) ---
      |                 |
-----                white(7/12)

      |
    white(7/12)----- black(5/12)
                  |
                    ----- White(7/12)

Now when we move to the right, we must multiply the chances and when we go up to down we must add the chances. Thus the chance for 1 black and 1 white = (5/12 * 7/12) + (5/12 * 7/12)

Now the same can be done for the isotopes if you know the percentage of being in nature and consider that as the chance to encounter that isotope in the compund. This also an experiment 'with putting back'

I hope this makes a little sence. It's pure maths.

Offline JZ_1

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Re:Mass spectra and Hydrates
« Reply #8 on: August 14, 2005, 06:17:55 PM »
This also an experiment 'with putting back'

What's a "putting back"?...or what does that mean?

-----------------------------------------------------------
Thus the chance for 1 black and 1 white = (5/12 * 7/12) + (5/12 * 7/12)

Now the same can be done for the isotopes if you know the percentage of being in nature and consider that as the chance to encounter that isotope in the compund. This also an experiment 'with putting back'

Isnt that what i'm trying to do with problem number 4 b)?
BUT i dont know the abundance...HOW can i find the abundance from the info posted from problem 4?

(68.926amu)*(??? %) + (70.926amu)*(??? %) = 69.72


Can i just put a "not enough Info" sign on that question?

if they didnt tell me what the abundance is...how am i suppose to find it??

well..i could use pure math..if i kno how…

Offline Donaldson Tan

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Re:Mass spectra and Hydrates
« Reply #9 on: August 14, 2005, 07:24:15 PM »
(68.926amu)*(??? %) + (70.926amu)*(??? %) = 69.72

let abundance of 69Ga be X
=> abundance of 71Ga = 1 - X

(68.926amu)*X + (70.926amu)*(1-X) = 69.72

solve for X
"Say you're in a [chemical] plant and there's a snake on the floor. What are you going to do? Call a consultant? Get a meeting together to talk about which color is the snake? Employees should do one thing: walk over there and you step on the friggin� snake." - Jean-Pierre Garnier, CEO of Glaxosmithkline, June 2006

Offline JZ_1

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Re:Mass spectra and Hydrates
« Reply #10 on: August 14, 2005, 07:35:09 PM »
yea..that's what i was talking about..

are u sure that it is 69Ga  and 71Ga

i kinda guessed it is those since the amu is the closest to those....

and to solve that..do i need to us edistrubutive property??..i forgot how to solve that....

i kno i have to take something out by dividing each side by something..

Offline JZ_1

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Re:Mass spectra and Hydrates
« Reply #11 on: August 14, 2005, 10:10:38 PM »
THX....i got that the abundance of 69Ga is 60.30% and 71Ga is 39.70%..right?

Forget about problem number 4....^_^

On to Problem number 5:
• An element X forms an iodide XI3 and a chloride XCl3. The iodide is quantitatively converted to the chlorine when it is heated in a stream of chlorine:
  2XI3 + 3Cl2 => 2XCL3 + 3I2

If 0.5000 g of XI3 is treated, 0.2360 g of XCl3 is obtained.

a) calculate the atomic weight of the element X.
b) Identify the element X.

So...how do i find the relationship between those two GRAMS????

What can i do to start this?…I dont think i can convert those two GRAMS to MOLES..since i dont kno what is the amu of the element X....

PLZ help..THX for the *delete me*!!

Offline Donaldson Tan

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Re:Mass spectra and Hydrates
« Reply #12 on: August 14, 2005, 10:25:41 PM »
are you familiar with algebra mathematics?

let M be the molar mass of element X

Quote
.how do i find the relationship between those two GRAMS????
2XI3 + 3Cl2 => 2XCL3 + 3I2

Can't you tell from the equation that the number of moles of XCl3 produced equal the number of moles of XI3 consumed?

Express the number of moles of XI3 consumed in terms of M
Express the number of moles of XCl3 produced in terms of M



« Last Edit: August 15, 2005, 01:42:16 PM by geodome »
"Say you're in a [chemical] plant and there's a snake on the floor. What are you going to do? Call a consultant? Get a meeting together to talk about which color is the snake? Employees should do one thing: walk over there and you step on the friggin� snake." - Jean-Pierre Garnier, CEO of Glaxosmithkline, June 2006

Offline JZ_1

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Re:Mass spectra and Hydrates
« Reply #13 on: August 14, 2005, 10:45:42 PM »
If 0.5000 g of XI3 is treated, 0.2360 g of XCl3 is obtained.

..dont comprehend...but why would they give me the
"0.5000 g of XI3  and  0.2360 g of XCl3"
the grams, if i dont need it...

Can't you tell from the equation that the number of moles of XCl3 produced equal the number of moles of XI3 consumed?

so..itz a one-to-one ratio..still about the grams....
how to convert them to moles??? how can they can be compared??
« Last Edit: August 14, 2005, 10:46:31 PM by JZ_1 »

Offline Donaldson Tan

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Re:Mass spectra and Hydrates
« Reply #14 on: August 15, 2005, 01:46:15 PM »
assuming 100% conversion,

molar mass of XI3 = M + 3(126.90) = M + 380.70 amu
number of moles of XI3 consumed = 0.500/(M+380.70)

molar mass of XCl3 = M + 3(35.5) = M + 106.5 amu
number of moles of XCl3 produced = 0.2360/(M+106.5)

0.500/(M+380.70) = 0.2360/(M+106.5)

solve for M
"Say you're in a [chemical] plant and there's a snake on the floor. What are you going to do? Call a consultant? Get a meeting together to talk about which color is the snake? Employees should do one thing: walk over there and you step on the friggin� snake." - Jean-Pierre Garnier, CEO of Glaxosmithkline, June 2006

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