Homework Help! (Chemistry)

[TSE]

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

[wvuwhat]

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



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

[ucfgrad93]

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.

[TSE]

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

[ucfgrad93]

Yes.

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

[gonyr]

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.

[furcalchick]

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

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.

[ucfgrad93]

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.

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

[gonyr]

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.

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.

[gonyr]

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

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.

[richard.mac]

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]

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


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.

[ucfgrad93]

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.

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

[Chundles]

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.

[furcalchick]

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.

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

[gonyr]

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.

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.

[ChrisA]

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.

[ChrisA]

edit: remove dup. (how to dups get in here?)

[Iscariot]

Avogadro
Avagadro
Avagadro

Also delicious in salad.

[richard.mac]

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.

my mistake… whats the formula?

i got the Forumla Unit definition from Wikipedia

[Clyde099]

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

What do you exactly do with one of those?

[ucfgrad93]

What do you exactly do with one of those?

Become somebody's spokesman.;)

[TSE]

Thanks for the help guys! Test on Friday. :(

[TSE]

New question!!!!!!

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

[Chundles]

New question!!!!!!

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

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

[furcalchick]

New question!!!!!!

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

is there any numbers given here? assuming you know the measurement of the gas in it's current form, you divide that by 22.4L (?) to get the mol.

[furcalchick]

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

that's pv=nrt. here's the wiki article (http://en.wikipedia.org/wiki/Pv%3Dnrt) for a quick refresher. there were three constants for p,v and t (pressure, volume and temperature in kelvin (i think)) in a certain situation. n is the gas constant and n is the number of moles.

[gonyr]

New question!!!!!!

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

The easiest method I can think of is to measure its density at STP. Then you can use the density in g/L X 22.4 L/mol to get a molar mass in g/mol. This works because 1 mol of any gas has a volume of 22.4 L at STP.

Alternately, you can measure the pressure, volume and temperature of a known mass of the gas. You will have P, V, and T from the ideal gas law (PV = nRT). You can solve for n, then use the mass of the gas / the number of moles (n) to get the molar mass.

[TSE]

STP was the correct answer guys! Thank you very much!

[furcalchick]

STP was the correct answer guys! Thank you very much!

i didn't think that was so easy as in just explaining the method. and yes, stp was the situation i was referring to in a previous post.

are you in high school or college btw? just asking so i can know what the difficulty of the exam is going to be on friday.

[smt1192]

Multiply the mols by the atomic mass.

[TSE]

I'm a sophomore in high school.

[furcalchick]

I'm a sophomore in high school.

those were the days. that's when i start learning about mols and the such. my teacher at least taught us a method that made mols a bit easier to calculate.

and since you have a big quiz on friday, i wonder if you're ready.

[TSE]

Yea, I'm not sure if I am I will be studying for 2 hours tonight. I will tell you guys how the test goes. Thank you all for your help and I am sure this thread will be used by me some more after this unit.

[TSE]

After studying for 3 hours of studying, practicing several equations, I GOT AN A!!!!!!!!!!!!!!!!!!!!!!!!!!! :D :D :D :D :D :D :D :D :D :D :D :D :D

[furcalchick]

After studying for 3 hours of studying, practicing several equations, I GOT AN A!!!!!!!!!!!!!!!!!!!!!!!!!!! :D :D :D :D :D :D :D :D :D :D :D :D :D

congrats!!