Understanding Moles: A Homework Challenge

Click For Summary

Discussion Overview

The discussion revolves around understanding the concept of moles in chemistry, particularly in the context of a homework problem involving the concentration of hydrogen ions when mixing nitric acid and sulfuric acid. Participants explore the definitions and implications of moles, atoms, and molecules, while also attempting to solve a specific concentration calculation.

Discussion Character

  • Homework-related
  • Conceptual clarification
  • Mathematical reasoning
  • Debate/contested

Main Points Raised

  • One participant expresses confusion over the statement that one mole is equivalent to one molecule and questions the teacher's assertion that one mole of methane contains five moles of atoms.
  • Another participant attempts to clarify the relationship between moles and molecules by comparing them to dozens, suggesting that one mole is analogous to a dozen but on a much larger scale (6.02 x 10^23).
  • Participants calculate the number of moles of hydrogen ions produced from the given volumes and concentrations of nitric and sulfuric acids, with varying interpretations of the results.
  • There is discussion about the treatment of diatomic molecules, with one participant questioning whether one mole of oxygen should be counted as one or two moles based on its diatomic nature.
  • One participant correctly identifies that while the number of moles can be added, concentrations cannot be directly summed without considering total volume.
  • Another participant suggests a final concentration calculation but is reminded that sulfuric acid is diprotic, indicating a need for further adjustment in their calculations.

Areas of Agreement / Disagreement

Participants demonstrate varying levels of understanding regarding the concept of moles and their application in the homework problem. There is no consensus on the correct interpretation of the teacher's statement or the final concentration calculation, indicating multiple competing views and unresolved questions.

Contextual Notes

Participants express uncertainty about the definitions and relationships between moles, molecules, and atoms, as well as the implications of diatomic molecules on calculations. Some calculations are presented without clear agreement on the assumptions or steps involved.

Who May Find This Useful

This discussion may be useful for students learning about stoichiometry, the concept of moles in chemistry, and the calculations involved in determining concentrations in solutions.

Celluhh
Messages
218
Reaction score
0

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.>.<
 
Physics news on Phys.org
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.
 
Borek said:
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]

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.
 
Celluhh said:
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?
 
Borek said:
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?
 
Celluhh said:
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.
 

Similar threads

Chemistry Why Should the Mole Ratio in Alkaline Buffers Be 5:1 Base to Acid?
  • · Replies 3 ·
Replies
3
Views
4K
Chemistry Moles of NaOH to Create Buffer Solution pH=6 in 0.42M Ethanoic Acid
  • · Replies 3 ·
Replies
3
Views
3K
Chemistry AvLaw - Which is the correct method of finding moles of gases?
  • · Replies 5 ·
Replies
5
Views
2K
Chemistry How to take into account autoprotolysis in weak base solution?
  • · Replies 8 ·
Replies
8
Views
3K
What is the percentage of Fool's Gold in the original sample?
  • · Replies 10 ·
Replies
10
Views
3K
Chemistry Relationship between Atomic PPM and Mole
  • · Replies 4 ·
Replies
4
Views
6K
Chemistry General Chemistry Moles Question
  • · Replies 9 ·
Replies
9
Views
3K
Chemistry Understand the Mole Concept: Formula of Compound w/ 3 O, 1 Al & 3 H
  • · Replies 3 ·
Replies
3
Views
2K
Chemistry How to Find Initial Moles of Reactants for Given Reaction | Moles Homework
  • · Replies 1 ·
Replies
1
Views
10K
Chemistry Calculating weight needed for micromole of solution
  • · Replies 1 ·
Replies
1
Views
2K