Mastering Concentration: Understanding How to Work Out Homework Answers

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Discussion Overview

The discussion revolves around understanding how to calculate concentrations in chemistry, specifically focusing on homework problems related to concentration calculations. Participants seek clarification on methods and equations used in these calculations, referencing specific examples from class materials.

Discussion Character

  • Homework-related
  • Mathematical reasoning
  • Conceptual clarification
  • Exploratory

Main Points Raised

  • One participant expresses confusion about how to work out answers related to concentration calculations after moving on to a new topic.
  • Another participant suggests a basic formula involving division and multiplication by 1000 for calculations.
  • Some participants mention the importance of unit conversions, specifically using dm-3 instead of liters.
  • There is a discussion about the approximation that 1 cm3 of solution equals one gram, particularly in the context of water.
  • A participant provides a detailed explanation of using dimensional analysis to convert from moles per cm3 to moles per dm3, illustrating the process with an example calculation.
  • Another participant expresses confusion about the explanation and requests a simpler breakdown of the process.
  • Some participants share their email addresses for further discussion and assistance outside the forum.
  • Examples of concentration calculations are proposed, including working out concentrations for given masses and volumes of solutions.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the best method for explaining the calculations, as some find the explanations confusing while others attempt to clarify. Multiple approaches and examples are presented, indicating a lack of agreement on a singular method.

Contextual Notes

Participants reference specific examples and conversion factors from class materials, but there is uncertainty regarding the accuracy of the calculations and the appropriateness of the approximations used. Some steps in the calculations remain unresolved or unclear.

Who May Find This Useful

This discussion may be useful for students learning about concentration calculations in chemistry, particularly those struggling with unit conversions and dimensional analysis.

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Homework Statement


I'm doing my best to figure out how these answers were worked out. We discussed it as a class but then moved on to another topic and after that, I just lost it because of the topic I'm doing right now.

I basically need to explain how these answers can be worked out.


Homework Equations



I have a screenshot of the powerpoint slide which was used and we were asked to calculate the answer. How would the answers be calculated?

con1how.jpg




The Attempt at a Solution



I have had an attempt at it and I was fairly good at this but once we started a new topic, I just lost all knowledge of it.

Please explain how I can work out the answers again.

Thanks!
 
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Does anyone know how we could get the answers in green?
 
first number divided by the second number multiply by 1000
 
thats for A
 
Yeah but is there something specific to remember or is there a specific equation to work through?
 
yeah its at the top in green. it tells you the unit conversions
 
so what they've done here is they've used dm^-3 instead of litre
 
1 cm^3 of solution is one gram
 
which is an approximation, because that is the case for H2O. sorry for like 30 posts...i don't know why i did this. rough day lol
 
  • #10
How would you work out the answer for A-3
How would you work out the answer for B-2
How would you work out the answer for C-1

Simple terms please.

Thanks. :)
 
  • #11
-------------------------------------------------------------------------------------


Ok, let's keep this Short and Sweet.

Basically this is what I need to do: "Examples of working through concentrations are shown"


Please help me ;(


I also have been given this slide and possibly I need to show how to calculate the answers from this too:

con2how.jpg
 
  • #12
Does anyone know?

Thanks.
 
  • #13
Any information will be appreciated which will enable me to understand it much more effectively on how these questions can be worked out.

Thanks.
 
  • #14
Dimensional analysis is key here. Look at the top in green, they give you the conversion factors. What we're looking for at the end is a concentration that is in moles/dm^3. What we have is (for a-3):
0.25 mol in 1000 cm^3 so there are 0.25 moles per 1000cm^3. So that's in moles per cm^3, we want to get to moles per dm^3.
So we have 0.25 mol/1000cm^3.
What we want is a concentration in moles/dm^3. So we need to find out how to get from moles/cm^3 to moles/dm^3. From the conversion factor at the top of the screen we know that 1000cm^3 = 1 dm^3. So here is where we would use dimensional analysis.
(0.25mol / 1000cm^3) * (1000cm^3 / 1dm^3) =
We use (1000cm^3 / 1dm^3) because the 1000cm^3 is in the denominator in (0.25mol / 1000cm^3). This way the units cancel out and we are left with moles/dm^3.

So if we go through the calculation...
(0.25mol / 1000cm^3) * (1000cm^3 / 1dm^3) =

The 1000's cancel out along with the units cm^3 and we are left with
(0.25mol / 1) * (1 / 1dm^3) = 0.25mol / dm^3.

Not enough time to help with the other two, but hope this helps you understand, cheers.
 
  • #15
Eeek!

That is very confusing.

Would you be able to add me on MSN or something? It seems you have a great range of knowledge about this stuff and I really need help on it.

Thanks.
 
  • #16
Let me show it in a simpler (less text) way. I tried to explain but maybe all the text is confusing you.

GIVEN: 0.25mol / 1000cm^3

WANT it in: mol / dm^3

CONVERSION factor(from top of slide): 1000cm^3 = 1 dm^3

We use 1000cm^3 / 1dm^3 in the calculation below because the GIVEN has cm^3 in the denominator so we need to use the CONVERSION factor with the cm^3 in the numerator.

So we take what we are GIVEN and multiple by the CONVERSION to get the answer.

(0.25mol / 1000cm^3) * (1000cm^3 / 1dm^3) = 0.25mol/dm^3I'm at school now and such and have class in a little bit, no msn on here, so I can talk when I get home but I won't get home till about 5 hours from now so... PM your address and we can chat if you're still having problems. Cheers.
 
  • #17
  • #18
are you on now, i got on through meebo
 
  • #19
Yep, I am online right now.
 
  • #20
What's your email address and I will add you too.
 
  • #22
I was speaking to you but you only said "?" and then logged out :P
 
  • #23
Would this be right for Activity 1?

Example: Work out the concentration of 20g of NaOH in a 250cm3 solution.

C = m ÷ V
m = 20g
V = 250cm3 ÷ 1000 = 0.25
NaOH = 20 x 0.25 = 5g dm-3
 
  • #24
Would this be right for Activity 2?

Example: Work out the concentration of a solution with 0.32 moles of NaCl in 300cm3.

C = n ÷ V
n = 0.32 moles
V = 0.32 ÷ 300cm3 = 0.02
NaCl = 0.02 x 1000 = 20 mol dm-3
 

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