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: Weight percent of element to give specific density for alloy

  1. Dec 2, 2008 #1
    1. The problem statement, all variables and given/known data
    Aluminium-Lithium alloys have been developed by the aircraft industry to reduce the weight and improve the performance of its aircraft. A commercial aircraft skin material having a density of 2.47 g/cm3 is desired. Compute the concentration of Li (in wt%) that is required.

    2. Relevant equations

    3. The attempt at a solution
    I've calculated the atoms/cm3 for both aluminium and Lithium, but i'm not sure how to relate them seeing as the question doesn't give an amount/quantity of either element. I assume there is an equation or something I should use but I can't think of it.
  2. jcsd
  3. Dec 3, 2008 #2


    User Avatar

    Staff: Mentor

    You have to assume that volumes are additivie. Imagine you have melted 1 mL of lithium with 1 mL of aluminum. Assume the volume of the alloy is 2 mL. What is its mass? What is its density? Do you see how it is related to the original question?

    Question is flawed, as volumes are not additive, but that's another story.
  4. Dec 4, 2008 #3
    Thanks, but i'm still not quite getting it. After adding the densities and dividing the wanted density by this amount I know it needs to be 1.617e+0 times more dense, but i still dont understand how to work this out. Obviously I have to increase the ratio of Al/Li but i'm not quite sure how i do this.
  5. Dec 5, 2008 #4


    User Avatar

    Staff: Mentor

    You may try with two equations - let VLi be volume of lithium, VAl be volume of aluminum used. Express total volume and density of alloy in terms of these two unknowns (that will give you two equations). Then solve. Assume total volume to be 2 or 7 or whatever, just give it some number value.

    It can be also done using volume fractions and in many other ways, but the one I have described is probably the most intuitive one.
    Last edited: Dec 5, 2008
  6. Dec 8, 2008 #5
    I think you might have to spell this one out for me Borek, I can't seem to get my head around it. I know you pretty much already have but it still isn't registering.
  7. Dec 8, 2008 #6


    User Avatar

    Staff: Mentor

    If you mix VLi and VAl of both metals - what will be total mass of the mixture?

    Assuming VLi + VAl = 2, what will be the density of the mixture?

    Do you see two equations in two unknowns?
  8. Dec 9, 2008 #7
    Sorry, i just can't see it. It is probably really simple, but i can't figure it out.
  9. Dec 9, 2008 #8


    User Avatar

    Staff: Mentor

    You should know densities of lithium and aluminum.

    What is mass of 1 mL of aluminum?

    What is mass of VAl milliliters of aluminum? (hint: use density definition)

    Do the same for VLi milliliters of lithium.

    Now you should know both masses and you know both volumes.

    What is total mass?

    What is total volume?

    Knowing mass of the alloy and volume of the alloy, use density definition to calculate alloy density.

    Go back to my previous post now.
  10. Dec 9, 2008 #9
    Ok, mass Al = 2.7VAl
    mass Li = .543VLi

    VAl + VLi = 2
    VAl = 2-VLi
    VLi = 2-VAl

    2.7(2-VLi) + .534(2-VAl) = 4.94

    1.528 = 2.7VLi + .534VAl

    I can then substitute VLi for 2-VAl and solve for VAl. I can then calculate VLi and subsequently calculate masses and weight percents of both.

    = 2.3wt% Li

    I think my problem was not seeing the big picture. E.g. work out both volumes and masses in terms of volumes first, and then incorporate them BOTH into the main equation.

    Thanks for your patience :).
    Last edited: Dec 9, 2008
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook