[Sleek]

can someone please help with #1 and 2?
im stuck.. i dont get what the question wants if there is no Cl there.. there's only C, C and O :/

[Borek]

No idea what your problem is, COCl₂ contains two chlorine atoms. You are not given masses of the chlorine isotopes, but that's what you should find from the given data.

[Sleek]

so the atomic mass of COCl₂ is 98.91 g/mol

"least abundant isotope of chlorine" means smallest value of atomic mass?
so i would use
13.003355 * 0.011 = 0.14304
15.994915 * 0.9976 = 15.95653

add them together and i get 15.9709
is that correct?

[Borek]

"least abundant isotope of chlorine" means smallest value of atomic mass?

No, least abundant is not connected to the atomic mass, but to the relative amounts of isotopes. Basically there are two, one dominates, the other is the least abundant.

[MOTOBALL]

Your spectrum has 5 isotopic peaks (3 with masses; 2 without masses given) in the molecular ion region shown.

You know that you have 12C, 13C, 16O, 35Cl and 37Cl present in COCl2.

Write out the molecular composition of each of the five, starting with m/z 97.932619

first, decide which is the minor isotope of chlorine---35Cl or 37Cl---then plug-in the given atomic weights into the appropriate molecular composition to find the atomic mass of 35Cl or 37Cl.

[Sleek]

ok i got 1 and 2. thanks for the *delete me*
for #3
it's %Cl in COCl₂
so [(36.9659*2) / 101.926721 ] *100
right?
i get 72.534, should i plug the answer as 73? cuz it says "try to get it within ±1%"

[DrCMS]

For #3 read the question again.

[Sleek]

"Enter the approximate % isotopic abundance of the least abundant isotope of chlorine present in the sample"
atomic mass of the least abundant isotope of chlorine =>
Cl₂: 36.9659*2 g
C: 12 g
O: 16 g
_________
101.93 g

74/102 = .73

i cant think of any other way to do it.. and the text book shows the same way too

[MOTOBALL]

You do not need the C and O present in phosgene.  The question relates only to the relative amounts of 35Cl and 37Cl (true also for ANY compound).

If you have n atoms of an element, which has two isotopes with "a" and "b" relative amounts, then the pattern in the molecular ion region of the mass spectrum is given by the binomial expansion,

[MOTOBALL]

       (a + b)ⁿ

Here there are n = 2 Cl atoms, so

first peak has intensity aⁿ
second peak               2 x a b
third peak                   bⁿ

So a² and b² = (a/b)² is the ratio of peak intensities for m/z 97.93 and 101.93.

Measuring from my monitor screen (you should do this more accurately with a mm ruler on a hardcopy) heights are,

69/16  and 7/16, ratio is 9.86 to 1; so take sq. root, then 3.14 to 1.

And 3.14 + 1 = 4.14 which is 100% of Cl atoms

so 35Cl is 3.14 and 37Cl is 1.0

so 1 ~ 100/4.14 = 24.15 %

If you measure more accurately, then you will get a more accurate determination.

Since you are doing this as an unknown, the instruction "try to get it +/- 1%" is somewhat pointless.

[Sleek]

"Measuring from my monitor screen.." im pretty sure this isnt what they expect us to do.. i have never done anything like this, ie using a ruler to find values for stuff in chem.
but even if using a ruler, how did you get 69/16  and 7/16?? i tried understanding where those fractions came from but didnt come up with anything :/

btw. this is the right answer but i just wanna know how to do it..

[MOTOBALL]

I literally put a ruler, with only 1/16 inch divisions, on the screen.

m/z 97 was 4 and 5/16 inches (69/16) high, while m/z 101 was 7/16 inches high.

You have the original hardcopy question---measure with a mm ruler !!!!!!!!!

You are using the ruler not to determine the absolute height of a peak, but rather to determine the RATIO of the two heights (intensities).

[Sleek]

"You have the original hardcopy question---measure with a mm ruler !!!!!!!!!"
i dont have the hardcopy. It's online questions. I dont think they expect me to print it out and measure it..
what confused me is that you used inches instead of cm.. thats all i was not clear about lol