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Finding the concentration of X

  1. Jun 29, 2011 #1
    1. The problem statement, all variables and given/known data
    How do I find the Density of X (Part A) and how do I find the Concentration of X in a solution (Part B)?

    Part A:
    I have 4mL of X. The mass of 4mL of X is 10g. Calculate the density of X.

    Part B:
    I have 7g of X in 100mL of Water. Calculate the concentration of X.

    The FINAL ANSWER must be written in BOTH "g/L" and as a percent (%).

    2. Relevant equations
    Density of X = Mass/Volume
    Concentration of X = grams of solute/Litre of solution

    3. The attempt at a solution
    What did I do:
    ------------------
    Part A:
    I have 4mL of X. The mass of 4mL of X is 10g. Calculate the density.

    Finding Density of X:
    I have 10g of X. (10g is the mass)
    I have 4mL of X. (I'm thinking that 4mL is the volume)

    Density of X = Mass/Volume
    Density of X = 10/4
    Density of X = 2.5g/mL

    Therefore, the density of X is 2.5g/mL.
    ------------------
    Part B:
    I have 7g of X in 100mL of Water. Calculate the concentration of X.

    Finding Concentration of X:
    I have 7g. (grams of solute)
    I have 100mL of Water, which is 0.100L. (Solution in L)

    Concentration of X = grams of solute/Litre of solution
    [X] = 7/0.100
    [X] = 70g/L

    As a percentage:
    70g/L = 70/1000 = 0.07 = 7%

    Therefore, the concentration of 7g of X in 100mL of Water is 70g/L (7%).

    -------------------------------------------

    Thoughts:
    You know how I calculated the density of X in Part A (2.5g/mL), don't I use that information to calculate the concentration of X in Part B?

    Does the density of the solute matter when calculating the concentration of X? If so, how do I use 2.5g/mL (density of X) in order to calculate the concentration of X in 100mL of Water?

    I hope you are able to help me.

    Thank you!
     
  2. jcsd
  3. Jun 29, 2011 #2

    ehild

    User Avatar
    Homework Helper
    Gold Member

    No, the density of the solute does not influence its concentration if its mass is known. Think, before adding x to the solvent, it can be solid, and in the solution the molecules are dispersed. No sense to speak of the density of x.

    You would need the density of x if its volume were given, to calculate the mass.

    ehild
     
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