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Calculate mass of compound given the mole % of total solution

  1. Nov 6, 2016 #1
    1. The problem statement, all variables and given/known data
    Known:

    Mass of compound A: 15mg (weighed in excess), MM=200g/mol
    Mass of compound B: 6mg, MM=700g/mol
    Concentration of each compound listed above(Concentration of solution of compound A or solution of Compound B): 20mg/L
    Total concentration of the solution: 20mg/mL
    Molar percent(percent of total number of moles in solution) of compound A: 20%
    Problem:
    I will list everything that is given to me by the problem as (Given:)

    So I measured a mass of compound A, lets say 15mg. And I measure a mass of compound B, lets say 6mg. (Given: concentration of each compound is 20mg/mL). Compound A is measured in excess. So using the mass, and the given concentration, I calculate the volume required for each compound(using the given concentration of 20mg/mL) and mix each compound into their required volumes to create 2 solutions. However, (Given: 10% molar percent[that is to say 10% of the total number of moles in the final solution]) of Compound A, what is the mass and volume of Compound A that I need to add to Compound B in order to get that mole percent? However, I cannot change the total concentration of the solution(Given: 20mg/mL)

    2. Relevant equations

    C=nv etc simple chem equations

    3. The attempt at a solution
    Moles A: 15mg/1000/200g/mol=7.5*10^(-5)
    Moles B: 6mg/1000/700g/mol=8.57*10^(-7)

    Total Moles in sol'n*(1-0.2)=Moles Compound B
    Total Moles in sol'n=1.07*10^(-6) moles
    Moles of Compound A needed=1.07*10^(-6) moles*0.2=2.14*10^(-7)
    Mass of Compound A=2.14*10^(-7)*200g/mol*1000=0.043mg
    Volume of Compound A to add=0.043mg/(20mg/mL)=0.00214mL
    ^This answer is wrong however.
     
  2. jcsd
  3. Nov 6, 2016 #2

    Borek

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    Staff: Mentor

    It is still not clear what you are trying to do and what you are calculating.
     
  4. Nov 6, 2016 #3
    Hi, I have two powders, compound A and compound B. I need to make 2 solutions each containing the two powders, so I use the concentration of each solution(20mg/mL) to determine how much of the solvent I need to add. Now I have 2 solutions with powder A and powder B dissolved in them. I need to add a certain amount of the compound A solution(since it was made in excess) to compound B solution so that the entire final solution has 10% molar mass of compound A. I am trying to calculate the mass Compound A and consequently the volume (as made in my solution containing compound A) that I have to add to the solution containing compound B . However, I must keep the final concentration of the solution at 20mg/mL. I hope this clarifies my question. Thank you!!
     
    Last edited: Nov 6, 2016
  5. Nov 7, 2016 #4

    Borek

    User Avatar

    Staff: Mentor

     
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