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lisanoce · May 18, 2009, 06:22 PM

How to calculate moles and concentration

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Equation: copper and silver acetate --> silver and copper acetate solution
so its Cu + AgCH3COO --> Ag + CuCH3COO right?

Mass of copper: 2.28g
Final mass of copper: 2.01 g (silver formed on it and was removed)

1) Calculate the # of moles of Cu(s) which reacted with the silver ions?
2) How many moles of silver ions were present in the 100-ml sample? What is the concentration of the silver ions in mol/L?
3) What is the concentration of acetate ions present, expressed in mol/L?

I need help solving these because it will be on my test : S thanks in advance!

Perito · May 18, 2009, 09:00 PM

$Cu + AgCH_3COO \rightarrow Ag + CuCH_3COO$

Mass of copper: 2.28g
Final mass of copper: 2.01 g (silver formed on it and was removed)

1) Calculate the # of moles of Cu(s) which reacted with the silver ions
2) How many moles of silver ions were present in the 100-ml sample? What is the concentration of the silver ions in mol/L?
3) What is the concentration of acetate ions present, expressed in mol/L?

Cu forms Cu+2. Ag forms Ag+. Your equation is not correct. These are the half-reactions that you can use to balance the equation.

$Cu \rightleftharpoons Cu^{+2}+2e^-$ (Equation 1)

$Ag^+ + e^- \rightleftharpoons Ag$ (Equation 2)

Acetate, $CH_3COO^-$ is a "spectator ion". It doesn't participate in the reaction. It may trade partners (silver to copper), but that's all. Treat it as a single entity.

To balance the half reactions, note that Cu gives up 2 electrons, while silver only accepts one. Multiply equation 2 by 2 and add the half reactions:

$Cu + 2Ag^+ \rightleftharpoons Cu^{+2} + 2Ag$ (Equation 3)

Add the acetate and you've got the balanced equation. Actually, equation 3 is sufficient to answer the questions.

a. Calculate the mass of copper that reacted from the two weights that you have. The beginning weight tells how much copper you started with; the ending weight tells how much did not react.
b. Divide the mass of copper that reacted by its atomic weight to get the number of moles of Cu that reacted.
c. From the balanced equation, you can tell how many silver atoms were deposited when one copper atom dissolved. Use this information to figure out how many moles of silver there were in the original solution.
d. The concentrations are pretty trivial once you know how much silver has reacted. All of the acetate came from the original silver that was present.

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