# Homework Help! (Chemistry)

Discussion in 'Community Discussion' started by TSE, Oct 27, 2008.

1. ### TSE macrumors 68030

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#1
I need help with homework! Can you please tell me how you got it?

Find the number of representative particles in each substance. (Use correct significant figures).

(a) 2.78 mol Sn
________atoms
(b) 0.590 mol KCl
________ formula units
(c) 7.49 mol SO2
________ molecules
(d) 5.91 multiplied by 10-3 mol NaI
________ formula units

2. ### wvuwhat macrumors 65816

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#2

This is precisely why I'm a Communication Studies major...

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#3
Well, a mole is equal to 6.023X10^23 atoms/molecules/particles.

So, multiplying that by 2.78 = 1.67X10^24, for the first question.

Hopefully, you can do the rest. Good luck.

4. ### TSE thread starter macrumors 68030

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#4
Wait so you multiply 2.78 by mol and that's the answer?

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#5
Yes.

Let me know what you get on your homework. Hopefully, I didn't lead you astray.

6. ### gonyr macrumors 6502

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#6
If you need to show your work (you probably do):

given # moles / 1 X 6.02 x 10^23 particles / 1 mole

BTW, all of your given measurements have three significant digits, so your answer should as well. My class has a test on mole problems tomorrow.

7. ### furcalchick macrumors 68020

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#7
actually you multiply it by Avogadro 's number (6.023x10(to the 23rd power)). same applies for c. btw.

for b and d, is formula units the molar mass of the molecule or something like that? if so, you get the mass of the molecule and multiply by the number of mols. correct me if i'm wrong btw.

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#8
Doesn't B only have 2? Its been awhile since I had to figure this out, so I could easily be wrong.

9. ### gonyr macrumors 6502

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#9
No, formula units is functionally equivalent to molecules. It's used for ionic compounds that aren't made of individual molecules like covalently bonded compounds. You would only use a formula mass if the question asked for the number of grams.

10. ### gonyr macrumors 6502

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#10
No, the zero at the end of a number, after the decimal point, is a measured digit. It shows the precision of the measurement. There's a difference in meaning between 0.5 and 0.5000.

11. ### richard.mac macrumors 603

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#11
atoms = Avagadro's number x mol

molecules = mol x molecular mass (m=n x M)

a formula unit is the empirical formula of an ionic or covalent compound and is the lowest whole number ratio of ions

so in this case as KCl and NaI are ionic and in lowest ratio, molecules = formula units

12. ### gonyr macrumors 6502

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#12
This is incorrect. The formula you have given will find mass (m), not the number of molecules.

Also, in my experience, formula unit is only used for ionic compounds, never covalent.

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#13
Ah, got it, thanks! Like I said it has been awhile, about 15 years.

14. ### Chundles macrumors G4

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#14

But seriously folks...

Get really comfortable with Avagadro's number, you'll have it etched in your brain by the time you're done with Moles. It's a bit difficult to get your head around but it's just another unit of measurement. Just like 6.02X10^23 carbon atoms is a mole of carbon, the same number of oranges is a mole of oranges. But, whereas we'd have no idea of how much a mole of oranges weighs, we're lucky in that one mole of any element's atoms will weigh exactly it's molar mass in grams.

15. ### furcalchick macrumors 68020

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#15
just double checking. didn't see formula units much.

16. ### gonyr macrumors 6502

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#16
A good way to think of it is as a counting unit, like a dozen, only much, much bigger. Just like a dozen is 12 of something, regardless of what the something is (like eggs, donuts, etc.), a mole is 6.02 x 10^23 pieces of something, regardless of what the thing is.

17. ### ChrisA macrumors G4

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#17
The key here is that a mol or ANYTHING always has the same number of particles. That is the whole point of the "mol". It makes the math really easy. Chemists invented the mol back when no one had calculators. Some smart guy figured out that if you used molls rather then grams you don't always have to divide or multiply buy some ugly number that you really don't know exactly.

Call the number o particles per mol "A" so then 2 mol has 2A particles and 2.435 mol has 2.435A particles and so on.

18. ### ChrisA macrumors G4

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#19

20. ### richard.mac macrumors 603

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#20
my mistake whats the formula?

i got the Forumla Unit definition from Wikipedia

21. ### Clyde099 macrumors regular

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#21
What do you exactly do with one of those?

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#22
Become somebody's spokesman.

23. ### TSE thread starter macrumors 68030

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24. ### TSE thread starter macrumors 68030

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#24
New question!!!!!!

How can you determine the molar mass of a gaseous compound if you do not know its molecular formula?

25. ### Chundles macrumors G4

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#25
Something about the Ideal Gas Law rings a bell but I'm not sure. Been years since they bestowed that bloody degree on me.