Density of sodium hydroxide

  1. Hi all..
    I have a question regarding the density of a solution.
    In my lab manual, it said to use 50 mL of 20% NaOH solution
    I have problem of understanding whether the 20% is the molar or mass or volume basis.. It doesn't state anything else in there.. what does it usually means? And i think i need density to find the number of moles of NaOH in this solution.. or am i wrong? :frown: :frown:

    Thanks in advance! :smile:
     
  2. jcsd
  3. iansmith

    iansmith 1,430
    Staff Emeritus
    Science Advisor
    Gold Member

    When solution are express in %, it is either volume per volume (v/v), weight per volume (w/v) or weight per weight (w/w). In the case, your solution is w/v and 10 g of NaOH are present in a 50 mL solution.

    All you need to do is calculated the number of mole and the molarity.
     
  4. Bystander

    Bystander 3,776
    Science Advisor
    Homework Helper
    Gold Member


    Reeaaallllllllyyyyy? Percentage when units are unspecified is w/w, and on very rare occasions, v/v (some liquid mixtures). Never w/v or v/w. It's sloppy usage to specify concentration in percentage --- chemists are sloppy. You'll run into all sorts of bizarre concentration scales, grains per MMF for odorizing natural gas, lbs. per gallon for pesticides, 3.2 beer (no one knows what the composition of the beverage really is) and in most such cases, the odd units are specified.
     
  5. Borek

    Staff: Mentor

    20% is either w/w or w/v, depending on the way it was prepared. If it is not precisely expressed it probably doesn't matter and NaOH is in large excess. In both cases you need density for precise calculations.

    20% w/w is 6.1M (d - 1.222)

    20% w/v is 5.0M (d = 1.187)
     
    Last edited by a moderator: Aug 13, 2013
  6. iansmith

    iansmith 1,430
    Staff Emeritus
    Science Advisor
    Gold Member

    We run into w/v all the time when it is not specify. I could show protocols in molecular biology that are filled with percentage with unspecify units and it is never. It is always either w/v and v/v.
     
  7. ShawnD

    ShawnD 986
    Science Advisor

    The assumption is usually the way you measure something. NaOH is a solid, so use mass. Water is a liquid, so use volume. NaOH in water would be m/v. If you take something like vodka which is alcohol in water, that would probably be v/v because alcohol and water are both liquids. Two salts would be m/m because they're both solids.
     
  8. The problem is that percentage solutions are ambiguous when units differ. For example, would a "20% NaOH solution" be 1 g NaOH/5 ml of H20? Or every 1 g NaOH/ 5 L or H20? Water happens to have a density of approximately 1 g/ml but what if you were using something like oil?

    If I saw something that said "20% solution" and it the solute wasnt a liquid, I would interpret it to mean that 20% of the solution consists of the solute. in other words, the mole ratio of solute to solvent is 20%. I would say that the directions are ambigous.
     
  9. ShawnD

    ShawnD 986
    Science Advisor

    You need to keep the units in the same ballpark. g goes with mL and kg goes with L. The only time you see things like g/L are with gases, and even those are just represented as (g/mL) x10^-3 unless otherwise specified. Oil is a liquid in which case you would use volume, so density really doesn't come into play.

    If it meant moles, it would say 0.20X; never 20%.
     
  10. i thought it was w/w.. and i couldn't find the density of NaOH 20% solution
    It makes more sense if it is w/v...

    Thanks a lot!!
     
Know someone interested in this topic? Share this thead via email, Google+, Twitter, or Facebook

Have something to add?