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Homework Help: Chemistry very easy please help I forgot how to do this

  1. Aug 2, 2010 #1
    1. The problem statement, all variables and given/known data

    For many years the recovery of gold-that is, the seperation of gold from other materials-involved the use of potassium cyanide:

    4AU + 8KCN + 02 +2H20 -> 4KAu(CN)2 + 4KOH

    What is the minimum amount of KCN in moles needed to extract 29.0 g (about an ounce) of gold?

    Now my question is one of the proper equations as I don't know what symbol to use for which quantity...

    I understand that

    mass = (molecular mass)(moles of mass)
    were mass has units of grams, g, symbol used curvy M, mass of each element in the compound times subscripts of the element i.e. C2 would be 2(12.00g) = 24.00 g
    mole of mass is the coeficent before the compound in the balanced reaction equation

    m = Mn

    I then would go to use subscripts were if the question asked about a compound on in part of the reactant balanced equation I would just use a R for a subscript and a Y for yield

    now this problem bothers me

    dividing the equation above by Mn I get

    m/(Mn) = 1

    and then go do the problem but sense the question is making reference to two reactants I had to use A and B for subscripts as well were A is the gold compound and B is the KCN compound

    (m_R_A)/( (M_R_A)(n_R_A) ) = (m_R_B)/( (M_R_B)(n_R_B) )

    solving for m_R_B

    ( (m_R_A)(M_R_B)(n_R_B) )/( (M_R_B)(n_R_B) ) = m_R_B

    now my probelm is that I know that m_R_A is given in the problem 29.0 g so I now that m = Mn solving for n I get m/M I need to solve for n so clearly then

    (m_R_B)/(M_R_B) = n_R_B = ( (m_R_A)(n_R_B) )/( (M_R_B)(n_R_B) )

    clearly my use of symbols is wrong but it makes sense were n_R_B is the is the 8 and also what the probelm is asking for me clearly a probelm but here is what I did

    ( (29.0 g Au)(8 mol KCN) )/( (197.0 (g Au)/(mol Au))(4 mol Au) ) = .294 mol KCN

    clearly this is a probelm even though its write because in the equation I said that KCN was 8 moles but when I solved I got .294 mol KCN are these two different values because they are both the amount of moles of KCN

    I'm kind of confused please let me know how to make sense of this in my equation and understand how they are different and the symbols I should use to indicate so as I don't know how to do this please let me know if you don't understand

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Aug 3, 2010 #2


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    Gold Member

    Don't make this more complicated than it has to be. Remember your equation:
    4AU + 8KCN + 02 +2H20 -> 4KAu(CN)2 + 4KOH

    You will see that for every mole of gold you need how many moles of cyanide? Can you tell from just looking at your equation? What is the ratio of gold/cyanide?
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