Homework Help: Concentration - Help please

1. Feb 2, 2010

LatexZone

1. The problem statement, all variables and given/known data
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.

2. Relevant 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?

3. 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.

Thanks!

2. Feb 2, 2010

LatexZone

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

3. Feb 2, 2010

dacruick

first number divided by the second number multiply by 1000

4. Feb 2, 2010

thats for A

5. Feb 2, 2010

LatexZone

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

6. Feb 2, 2010

dacruick

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

7. Feb 2, 2010

dacruick

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

8. Feb 2, 2010

dacruick

1 cm^3 of solution is one gram

9. Feb 2, 2010

dacruick

which is an approximation, because that is the case for H2O. sorry for like 30 posts...i dont know why i did this. rough day lol

10. Feb 2, 2010

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

Thanks. :)

11. Feb 2, 2010

LatexZone

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Ok, lets keep this Short and Sweet.

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

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

12. Feb 3, 2010

LatexZone

Does anyone know?

Thanks.

13. Feb 3, 2010

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.

14. Feb 3, 2010

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.

15. Feb 3, 2010

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.

16. Feb 3, 2010

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^3

I'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. Feb 3, 2010

LatexZone

18. Feb 3, 2010

jahrollins

are you on now, i got on through meebo

19. Feb 3, 2010

LatexZone

Yep, I am online right now.

20. Feb 3, 2010

LatexZone

21. Feb 3, 2010

jahrollins

22. Feb 3, 2010

LatexZone

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

23. Feb 3, 2010

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

24. Feb 3, 2010

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