[dk_]

there are a lot of smart ppl on soompi so plz help me with these types of mole problems.. i just don't get it

How many hydrogen atoms are in 5.0 moles of water or

How many hydrogen atoms are in 2.1 moles of ammonia... like it seems so simple but i dunno how to get it i just figured out the following
NH4
Element # of atoms average atomic mass
N 1 14.01g/mole
H 4 1.008g/mole
so N there is 14.01 x 1= 14.01g/mole
and H there is 1.008x4= 4.032g/mole

then you add it all up and you get 1.80 x 10g/mole... and then heres where i get stuck.. i don't know what to do anymore how am i supposed to know just the hydrogen? i know that if you multiply the compound by avogadros number (6.022x10^23mol^-1) that you get the # of molecules.. but im not sure what to do when the elements are 'separated'
please help ive been stuck on this problem for a while thanks a lot

[yummers]

omgsh i got so mad when i saw this cos i remember doing it ages ago in yr 11 and have totally forgotten all of it!
i dunno maybe you should try work it out by percentage by mass or something, like M(NH4) = 14.01+( 4*1.008) = 18.042M
since M(H) = 1.008M (sorry i cant remember clearly but M is g/mol yeah?)
then percentage of H in NH4 is (4*1.008)/18.042 = 0.223*100 = 22.35%
so to work out n(H), n(H) = n(NH4)*0.223 = 2.1*0.223 = 0.469mol
so the amount of H atoms is 0.469*6.023*10^23 = 2.8*10^23 atoms
remember i havent done this for ages and COULD BE WRONG! lol, maybe you should check the answer with my answer to make sure cos i seriously am rusty on this

[mtrgl7]

erm im pretty sure you dont need to find the masses of N and H.

i dont really understand your working but judging by your question

How many hydrogen atoms are in 5.0 moles of water or

How many hydrogen atoms are in 2.1 moles of ammonia

5 moles of H2O means 5 x 6.02x10^23 molecules of H2O. each molecule of H2O has 2H atoms right? so 5 moles of H2O has 5 x 6.02x10^23 x 2 of H atoms. you understand that?

its the same for NH3.

2.1 moles NH3 = 2.1 x 6.02x10^23 molecules of NH3. each molecule of NH3 has 3H atoms. so 2.1 moles NH4 has 2.1 x 6.02x10^23 x 3 of H atoms.

you do the math. hope i helped. good luck with chemistry

[boka]

^ sounds right... it's so easy to forget this stuff - i worked in a lab for a year and had to google old formulas and constants every time i had to mix up some krebs solution

[Malice_Kaiser]

Wow. I just took a test on this exact stuff literally yesterday, and I still don't know how to do it.

[Srey Mao]

I did this in like October.. I don't remember anything. I failed all the tests and quizzes on this subject, but now I'm doing pretty good at whatever I'm learning now!! AHHAHA.

good luckkk!

[boltar]

How many hydrogen atoms are in 2.1 moles of ammonia... like it seems so simple but i dunno how to get it i just figured out the following

yup... i agree with mtrgl7's solutions. you don't need anything dealing with mass to solve this problem.

also, ammonia is NH3 and ammonium is NH4... just nitpicking.

[ChunJin]

Here's a good tip for future-keep sake.

It's a good idea to do this little chart-trick type thing on relationships.

Like this ->

1 kilogram | 1000 grams
-------------| 1 kilogram

Kilograms cancels out leaving grams.

It's a little chart where you cross out and it leaves you with what you desire. It helps out quite a bit; it's just a chart of relationships.

What's below it is the relationship above it. Like 1 kilogram = 1000 grams; so you put them on top of each other depending on the situation.

1 gram | 1 kilogram
---------| 1000 gram

grams cancels out leaving kilograms.

5.0 mol H2O | 6.022 x 10^23 H2O Atoms | 2 H Atoms [contains 2 Hydrogen]
----------------| 1 mol H2O --------------------| 1 H2O Atoms

mol H2O cancels out, and H2O Atoms cancels out.

2.1 mol NH3 | 6.022 x 10^23 NH3 Atoms | 4 H Atoms
----------------| 1 mol NH3 --------------------| 1 NH3 Atoms

[mtrgl7]

^ oh man i really have no idea what you mean. whats the canceling out thing? i dont understand how it works.

[ChunJin]

Basically, you setup a relationship previous to the last one.

Say you wanted to convert 48 minutes into seconds. We know that 1 minute = 60 seconds and we want to get to seconds. So we setup a relationship.

48 minutes | 60 seconds
-------------| 1 minute

[trixtah]

just to make clear: atoms, moles whatever, in ammonia (NH3) theres 1 (atom/mol) Nitrogen and 3 (atoms/moles) Hydrogen. you can convert it while in moles, or after

edit: actually, mtrgl and you have it wrong. ammonia is NH3, ammonium(ion)is NH4+

so alternatively, you can go from 2.1 moles NH3, 3 moles Hydrogen per 1 mole NH3, 6.022x10^23 atoms per 1 mole Hydrogen (or anything else for that matter)

when doing stoichiometry like this, there's no real adding, just conversions



REMEMBER YOUR SIG FIGS! (you can see I almost forgot hehe)

[GDxPassion]

Wow! I remember doing this in AP Chemistry my sophomore year in high school...can't believe I still remember how to do it! ahaha Stinkin' moles and molarity...

[mtrgl7]

just to make clear: atoms, moles whatever, in ammonia (NH3) theres 1 (atom/mol) Nitrogen and 3 (atoms/moles) Hydrogen. you can convert it while in moles, or after

edit: actually, mtrgl and you have it wrong. ammonia is NH3, ammonium(ion)is NH4+

so alternatively, you can go from 2.1 moles NH3, 3 moles Hydrogen per 1 mole NH3, 6.022x10^23 atoms per 1 mole Hydrogen (or anything else for that matter)

when doing stoichiometry like this, there's no real adding, just conversions

yeah mistake up there. i didnt realised it. edited the solution already.

[oxymoronic]

2.1 mol NH4 | 6.022 x 10^23 NH4 Atoms | 4 H Atoms
----------------| 1 mol NH4 --------------------| 1 NH4 Atoms

the aim of the chart is to equate/balance things until youre left with the UNIT that you want. in this case^^ hydrogen.
as long as there is the unit somewhere on the bottom and the op of the chart it will cancel out.

so:
2.1 mol NH4 << needs to cancel out mol NH4
put 1 mol NH4 on the bottom of the chart in the next column so it cancels out the mol Nh4 on top
1 NH4 equals (on the top) 6.022etc. NH4 atoms << need to cancel out that unit (NH4 atoms)
put 1 NH4 atom on the bottom to cancel out the NH4 atoms on the top then continue

do you get it? once you've finished balancing that out, then you multiply all the top numbers and then divide by the product of the bottom numbers and your unit is the unit that isnt canceled out (aka doesnt appear on both top and bottom)

sorry if that was no help....

[ChunJin]

Err =p I just followed the last person; it should be NH3 since it's ammonia [not ammonium].

I probably should have done -> 6.022 x 10^23 Molecules; then in 1 molecule of NH3 there's 3 atoms of H.

2.1 mol of NH3 | 6.022 x 10^23 Molecules of NH3 | 3 Hydrogen Atoms
-------------------| 1 mol of NH3 --------------------------| 1 Molecule of NH3

Just leaving atoms.

[lovablekrnstar63]

looks like you got your answer
i got my paper out and started doing this
but i saw everybody else that posted so..
(stoichiometry was so easy for me.. but i still got an A- on the test a couple of months ago T.T)

[pandazombie]

ahh i remember this..stoichiometry or something but the teacher called it the fence post thing XD


it's just conversion..like what other people posted up

it's not that hard if you get the hang of it..

[dk_]

i asked someone whos in my school how to do this and they just told me you multiply
#of moles * # of atoms (there are 3 in ammonia)*avogadros number
so in How many hydrogen atoms are in 2.1 moles of ammonia?

2.1 moles * 3 * 6.022*10^23
=3.8 x 10^24 atoms...


im not sure why that works out or anything but thanks for all your responses... anyone know that method?

[trixtah]

i asked someone whos in my school how to do this and they just told me you multiply
#of moles * # of atoms (there are 3 in ammonia)*avogadros number
so in How many hydrogen atoms are in 2.1 moles of ammonia?

2.1 moles * 3 * 6.022*10^23
=3.8 x 10^24 atoms...
im not sure why that works out or anything but thanks for all your responses... anyone know that method?

that person isn't very bright. Never just multiply things like that without writing it out to make sure the conversions cancel out. That works only because (look above at my work). He left out the denominator AND the units in that. Everything cancels out until you get the number of atoms. Normally, a teacher will mark you off for leaving off units and not showing the converting steps.

if you follow that guy's work, you get 3.8 x 10^24 atoms*moles*atoms/moles which makes no sense at all. Just look over what people have said, I don't think you understand this topic :/ but don't worry, it's easy once you catch on

Here is an example of a simple problem using the exact same concept kind of like someone posted above



edit: since I don't want to leave you confused, here is a quick problem I made up before I go to bed



(i hope i didnt mess up...did this really quickly zzzz)

[ChunJin]

I like that approach and it works but it's a bit *confusing in how it's setup* ... because well you have mol/mol. You don't ever want that.

I'd rather do ->

grams -> (mol / gram) -> (molecules / mol) -> (atoms / molecules). Grams cancel out with grams, mol with mol, molecules with molecules leaving atoms. You don't have a ratio on itself.

Seeing how you don't have a mol/mol situation which makes it cancel itself out heh.