Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Molar Solutions with percent concentrations

  1. Sep 12, 2010 #1
    1. The molecular weight of NaCl is 58.54. A 1 M solution of NaCl has 58.54 g NaCl dissolved in distilled water to a total volume of 1000 mL or 1 L. a .1 M solution has .854 g/L. A .2 M solution has 11.71 g/L.

    Qustion 1 - What is the molarity of a 5% w/v NaCl solution?

    Question 2 - What is the percent concentration of a 2M NaCl solution?

    Question 3 - The molecular weight of glucose is 180.16. Describe how you would make up 100 mL of a .5M solution.

    2. The only equation I have come up with is Molarity*Volume = grams/molecular weight. But I have no clue if that is relevant or what to do with it.

    3. All I have been able to do is verify what the information gives me such as:

    1 x 1 = 58.54/58.54

    .1 x 1 = 5.84/58.54

    .2 x 1 = 11.71/58.54

    But I don't know where percentage comes into this at all. I haven't been able to find any examples on the internet that show me how to even begin solving these questions.
  2. jcsd
  3. Sep 12, 2010 #2


    User Avatar
    Homework Helper
    Gold Member

    The 0.1 M solution must be 5.854 g/L not .854 g/L.

    Do I need to explain that 5% w/v means 5 g/L?

    You can do the first two questions without even knowing what M or molecular weight even mean, by simple proportions.

    Though you cannot get anywhere in chemistry without knowing what they mean - you must surely find something in your textbook, course notes or internet.
  4. Sep 12, 2010 #3
    Sorry, yes it is 5.854 g/L.

    For the first question would the answer be .08 M then? The second question confuses me though...I get 117.08 as the answer but how does that make sense as a percentage?

    And for whatever reason I've got to do this worksheet for my biology class...I'm not sure why and the last time I studied chemistry was 4 years ago so I have very little idea about what all of this is and my teacher didn't give us any instruction other than to complete it.

    Thanks for the response.
  5. Sep 12, 2010 #4


    User Avatar
    Homework Helper
    Gold Member

    I am sorry, of course 5% w/v means 5g/100 ml i.e. 50g/l
  6. Sep 12, 2010 #5
    I'm really confused.

    For question one, does:

    M = (50 g/l)/(58.54)

  7. Sep 12, 2010 #6


    User Avatar
    Homework Helper
    Gold Member

    Even correcting for that mistake your first answer to Q1 would not have been right.

    For you second answer 117.08 is the right number, but pure numbers do not mean anything in this sort of question. It is 117.08 g/l. Not hard to work out how many g per 100 ml which is the w/v %. Again quote the units.

    And since your information was given to 4 significant figures, the answers can be too.

    Your teacher probably thinks the relevant concepts are so elementary that even someone who has forgotten most of his chemistry still knows them, so I recommend you revise moles, molarity and why they are used.
  8. Sep 12, 2010 #7


    User Avatar
    Homework Helper
    Gold Member

    Sort of, the molarity is 50/58.54, the solution is (50/58.54) M.
  9. Sep 12, 2010 #8
    So for question one the answer is 50/58.54 M, which is .854 M.

    For question two, I did (117.08 x 100)/1000 and got 11.7%.

    For the third question I did:

    (100/1000)*(.5) = x/180.16

    and got 9 grams(?). So would that mean dissolving 9 grams of glucose in the 100 mL of solution? Or am I completely wrong?
  10. Sep 12, 2010 #9


    User Avatar
    Homework Helper
    Gold Member

    Q1 is right.

    also agree with Q2

    Q3 9 g near enough. I get 9.01 g .

    Not completely wrong - wording makes me not sure what you meant. You are making the solution by dissolving the glucose in water not in solution. What you should do to be accurate is add water to your glucose until the total volume is 100 ml. It says w/v - weight/volume. That does not make exactly 100 ml of water, though not far off. You can't practically calculate how much water, but you don't need to.
  11. Sep 12, 2010 #10
    Ok I think I understand it all now. Thank you for all of your help!
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook