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Homework Help! (Chemistry)

TSE

TSE

macrumors 601
Original poster
Jun 25, 2007
4,105
3,747
St. Paul, Minnesota
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
 
TSE said:
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
Click to expand...



This is precisely why I'm a Communication Studies major...
 
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.
 
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.
 
TSE said:
Wait so you multiply 2.78 by mol and that's the answer?
Click to expand...

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.
 
gonyr said:
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.
Click to expand...

Doesn't B only have 2? Its been awhile since I had to figure this out, so I could easily be wrong.
 
furcalchick said:
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.
Click to expand...

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.
 
ucfgrad93 said:
Doesn't B only have 2? Its been awhile since I had to figure this out, so I could easily be wrong.
Click to expand...

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.
 
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
 
gonyr said:
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.
Click to expand...

Ah, got it, thanks! Like I said it has been awhile, about 15 years.:D
 
Isn't the answer just "Jesus"?

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.
 
gonyr said:
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.
Click to expand...

just double checking. didn't see formula units much.
 
Chundles said:
Isn't the answer just "Jesus"?

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.
Click to expand...

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.
 
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.
 
New question!!!!!!

How can you determine the molar mass of a gaseous compound if you do not know its molecular formula?
 
TSE said:
New question!!!!!!

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

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