Understanding Moles: A Homework Challenge

[Celluhh]

Homework Statement

i have just learned that one mole is actually what i had learned as one molecule. now one atom is also known as one mole.eg. one mole of methane has 5 moles of atoms( this sentence is written by my teacher and no matter how i try to look at it from different perspectives it just seems wrong, i can't seem to understand it.:( )

anyway, this is one of the homework qns i have problems with.

what is the concentration of hydrogen ions when 100cm3 of 0.20 mol/dm3 of nitric acid is mixed with 300cm3 of 0.25mol/dm3 sulfuric acid?

Homework Equations

HNO3 ---> H+ + NO3-

The Attempt at a Solution

can someone help me solve this and explain to me how to solve it? I'm also trying to solve it right now.>.<

 

[Borek]

One dozen of methane molecules is 12 molecules, right?

One methane molecule - CH₄ - is five atoms, one carbon and four hydrogens, right?

So one dozen of methane molecules contains - together - 5 dozen of atoms of all kinds, right?

Now replace "dozen" with a "mole" and repeat whole exercise.

Please show how you approached the H⁺ question. It is not difficult - it is almost a direct application of the concentration definition, just both volume and number of moles are not given directly, but have to be calculated.

Actually it IS a difficult question, but for reasons you don't know yet. Don't worry, you are expected to ignore 90% of what is really happening in the solution. At the same I would NEVER ask such a question. I hate questions that are wrong by definition.

 

[Celluhh]

One dozen of methane molecules is 12 molecules, right?

One methane molecule - CH₄ - is five atoms, one carbon and four hydrogens, right?

So one dozen of methane molecules contains - together - 5 dozen of atoms of all kinds, right?

Now replace "dozen" with a "mole" and repeat whole exercise.

[COLOR][/SIZE]

um ok i get the dozen thing but i don't get why i should replace "dozen"with "molë"?

uh ok firstly i found the number of moles in the nitirc acid--- which is 1/10 dm3* 0.20mol/dm3 which is equal to 0.02 mol
since the no. of mol in the nitric acid is 0.02, conc of h ions is 0.02 as well.

no. of moles in sulphuric acid is 0.075 so conc of h ions is 0.15. conc of h ions altogether is 0.15 + 0.02 which is equal to 0.17??

ok this seems wrong somehow.

 

[Borek]

um ok i get the dozen thing but i don't get why i should replace "dozen"with "molë"?

Mole is an overgrown dozen. Dozen is 12 objects, mole is 6.02x10²³ objects.

uh ok firstly i found the number of moles in the nitirc acid--- which is 1/10 dm3* 0.20mol/dm3 which is equal to 0.02 mol
since the no. of mol in the nitric acid is 0.02, conc of h ions is 0.02 as well.

no. of moles in sulphuric acid is 0.075 so conc of h ions is 0.15. conc of h ions altogether is 0.15 + 0.02 which is equal to 0.17??

You started right - you correctly calculated number of moles of H⁺ from both sources. However, while you can add numbers of moles, you can't add concentrations.

You know how many moles of H⁺ from each source you have. Can you calculate total number of moles? Do you know final volume? Can you calculate concentration using both number of moles and volume?

 

[Celluhh]

Mole is an overgrown dozen. Dozen is 12 objects, mole is 6.02x10²³ objects.



?

yeah i know avogadro's constant. but i still don't understand why?? ok another qns, oxygen is diatomic, but it is counted as one mole not 2 right? (ps. am i making any sense??)

oh so i should take total no. of moles over total volume to get conc of hydrogen ions which is 0.095mol/ 0.4dm3 which gives me 0.238mol/dm3 (3s.f.)

is this right?

 

[Borek]

yeah i know avogadro's constant. but i still don't understand why??

No idea what the question is. Why what? Why you can replace "dozen" with a "mole"? Because both refer to the same - number of objects. Just like gross (dozen of dozens, or 144 objects) or threescore (60 objects). Numbers are different, but the idea is identical.

ok another qns, oxygen is diatomic, but it is counted as one mole not 2 right? (ps. am i making any sense??)

It counts as one mole of diatomic molecules, or two moles of atoms. Usually when we say "mole of oxygen" we mean "mole of diatomic molecules".

oh so i should take total no. of moles over total volume to get conc of hydrogen ions which is 0.095mol/ 0.4dm3 which gives me 0.238mol/dm3 (3s.f.)

Close, but you forgot that sulfuric acid is diprotic.