# Stuck on chemistry MCQ HELP!

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#1
I've tried combing the two equations together giving:

2Cu2 + 4I- + 2S2O3 2- ----> 2CuI + 2I- + S4O6 2- (where the I2 on both sides of the equation cancel out)

I've also found moles of the sodium thiosulfate from using mol = conc * vol so (24.5/1000) * (0.100) = 0.00245 moles

Not entirely sure what to do next now?
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1 year ago
#2
Use the mole ratio to find the number of moles and hence mass of copper

Find %
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#3
(Original post by BobbJo)
Use the mole ratio to find the number of moles and hence mass of copper

Find %
So from the combined equation I got, I know the molar ratio to be 1:1 between sodium thiosulfate and copper iodide.

And 1 mole of CuI has 1 mole of copper

So mass of copper is '0.00245 * 63.5 = 0.155575 g'

Hence to find percentage purity for copper, it's just 0.305/0.155575 * 100 = 51%

Does this seem okay?
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1 year ago
#4
(Original post by Yatayyat)
So from the combined equation I got, I know the molar ratio to be 1:1 between sodium thiosulfate and copper iodide.

And 1 mole of CuI has 1 mole of copper

So mass of copper is '0.00245 * 63.5 = 0.155575 g'

Hence to find percentage purity for copper, it's just 0.305/0.155575 * 100 = 51%

Does this seem okay?
Yes but wrong logic. (correct answer though)

We have Cu2+

It reacts with the iodide from potassium iodide to give CuI and I2

From the first equation, 2 moles of Cu2+ give 1 mole of I2

then the I2 liberated is titrated with thio.

2 mole of thio reacts with 1 moles of I2

Then overall it's 2 moles of Cu2+ reacting with 2 moles of thio and that is the 1:1 ratio

It is not the moles of CuI that should be considered, but moles of Cu2+. (which are equal so the answer is the same)
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#5
(Original post by BobbJo)
Yes but wrong logic. (correct answer though)

We have Cu2+

It reacts with the iodide from potassium iodide to give CuI and I2

From the first equation, 2 moles of Cu2+ give 1 mole of I2

then the I2 liberated is titrated with thio.

2 mole of thio reacts with 1 moles of I2

Then overall it's 2 moles of Cu2+ reacting with 2 moles of thio and that is the 1:1 ratio

It is not the moles of CuI that should be considered, but moles of Cu2+. (which are equal so the answer is the same)
I'm sort of getting it, but where are you getting the 2 moles of I2 from?
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1 year ago
#6
(Original post by Yatayyat)
I'm sort of getting it, but where are you getting the 2 moles of I2 from?

I did not write 2 moles of I2
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#7
(Original post by BobbJo)

I did not write 2 moles of I2
Sorry, yeah I did accidentally misread it, I understand it now. Thank you for clearing up my misconception.
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