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Homework Help: Concentration - Help please

  1. Feb 2, 2010 #1
    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.

    Please explain how I can work out the answers again.

  2. jcsd
  3. Feb 2, 2010 #2
    Does anyone know how we could get the answers in green?
  4. Feb 2, 2010 #3
    first number divided by the second number multiply by 1000
  5. Feb 2, 2010 #4
    thats for A
  6. Feb 2, 2010 #5
    Yeah but is there something specific to remember or is there a specific equation to work through?
  7. Feb 2, 2010 #6
    yeah its at the top in green. it tells you the unit conversions
  8. Feb 2, 2010 #7
    so what they've done here is they've used dm^-3 instead of litre
  9. Feb 2, 2010 #8
    1 cm^3 of solution is one gram
  10. Feb 2, 2010 #9
    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
  11. Feb 2, 2010 #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. :)
  12. Feb 2, 2010 #11

    Ok, lets 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:

  13. Feb 3, 2010 #12
    Does anyone know?

  14. Feb 3, 2010 #13
    Any information will be appreciated which will enable me to understand it much more effectively on how these questions can be worked out.

  15. Feb 3, 2010 #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.
  16. Feb 3, 2010 #15

    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.

  17. Feb 3, 2010 #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^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.
  18. Feb 3, 2010 #17
  19. Feb 3, 2010 #18
    are you on now, i got on through meebo
  20. Feb 3, 2010 #19
    Yep, I am online right now.
  21. Feb 3, 2010 #20
    What's your email address and I will add you too.
  22. Feb 3, 2010 #21
  23. Feb 3, 2010 #22
    I was speaking to you but you only said "?" and then logged out :P
  24. Feb 3, 2010 #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
  25. Feb 3, 2010 #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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