Mastering Concentration: Understanding How to Work Out Homework Answers

[LatexZone]

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?

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!

 

[LatexZone]

Does anyone know how we could get the answers in green?

 

[dacruick]

first number divided by the second number multiply by 1000

 

[dacruick]

thats for A

 

[LatexZone]

Yeah but is there something specific to remember or is there a specific equation to work through?

 

[dacruick]

yeah its at the top in green. it tells you the unit conversions

 

[dacruick]

so what they've done here is they've used dm^-3 instead of litre

 

[dacruick]

1 cm^3 of solution is one gram

 

[dacruick]

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

 

[LatexZone]

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. :)

 

[LatexZone]

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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:

 

[LatexZone]

Does anyone know?

Thanks.

 

[LatexZone]

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

Thanks.

 

[jahrollins]

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.

 

[LatexZone]

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.

 

[jahrollins]

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.

 

[LatexZone]

I think I'm starting to get it.
A little more easy examples and I will be on the way :)

My MSN: samschofield1993@hotmail.co.uk

Thanks.

 

[jahrollins]

are you on now, i got on through meebo

 

[LatexZone]

Yep, I am online right now.

 

[LatexZone]

What's your email address and I will add you too.

 

[jahrollins]

lordofgoodandevil@msn.com

 

[LatexZone]

I was speaking to you but you only said "?" and then logged out :P

 

[LatexZone]

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

 

[LatexZone]

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