# Redox stoichiometry

1. Oct 24, 2009

### Suy

1. The problem statement, all variables and given/known data

a 0.245 mol/L silver chloride solution is mixed with 50.0 mL of 0.200 mol/L zinc iodide solution. what volume of silver chloride solution is needed for a complete reaction?
2. Relevant equations

3. The attempt at a solution
Ag+ is the strongest OA,
I- is the strongest RA
2Ag+2e-->2Ag(s)
2I- -->2e+I2
2Ag+2I- -->2Ag(s)+I2
0.200*(50/1000)=0.01
0.01/2 *2=0.01
0.01/0.245=

40.8mL

But the answer is 81.6mL, which part is wrong??
ty!

Last edited: Oct 24, 2009
2. Oct 24, 2009

### Bohrok

2I- -->2e+2I2

That 2 doesn't belong.

3. Oct 24, 2009

### Suy

yes, but the answer still wrong though...

4. Oct 24, 2009

### Bohrok

0.01/2 *2=0.01
I don' see why you're dividing by 2 above.

Show your units with the numbers and it'll be easier to find your mistake.

5. Oct 24, 2009

### Suy

2Ag+2e-->2Ag(s)
2I- -->2e+I2

0.01mol / 2mol I-(aq) * 2mol Ag(aq)
=0.01mol

0.01mol / 0.245mol/L
=0.0408L
=40.8mL

also, if i use 2AgCl + ZnI2 ---> 2AgI + ZnCl2
i can get 81.6mL, but this question is in my redox unit....
or is this a trick question?
thx

Last edited: Oct 24, 2009
6. Oct 24, 2009

### Bohrok

I think the error is here: 0.200*(50/1000)=0.01
The 0.200 is the molarity of ZnI2, but the molarity of the I- ions is actually twice that much since there are 2I- in that compound. So you should use a molarity of 0.400 for the I-.

You could use the net ionic/half-reaction equation 2Ag+ + 2I- → 2Ag + I2 (or the actual equation which I believe is 2AgCl + ZnI2 → 2Ag + ZnCl2 + I2) and use conversion factors to find the answer; I got the correct answer doing it this last way.

Last edited: Oct 24, 2009
7. Oct 25, 2009

### Suy

But it's doesn't make sense to me if the I- ions s twice
I- -->1e+ (1/2)I2
then 0.01mol will be the same, since
Ag+1e-->1Ag(s)
I- -->1e+ (1/2)I2
0.01mol/ 0.245mol/L,
or is the textbook answer is wrong??

8. Oct 25, 2009

### Bohrok

$$\frac{0.200 ~mol~ ZnI_2}{1~ L~ZnI_2~solution} \rightarrow \frac{0.400~mol~I^-}{1~L~ZnI_2~solution}$$

$$\frac{0.245~mol~AgCl}{1~L~AgCl~solution} \rightarrow \frac{0.245~mol~Ag^+}{1~L~AgCl~solution}$$

Using 2Ag+ + 2I- → 2Ag + I2,
$$0.0500~L~ZnI_2~solution\left(\frac{0.400~mol~I^-}{1~L~ZnI_2~solution}\right)\left(\frac{2~mol~Ag^+}{2~mol~I^-}\right)\left({\frac{1000~mL~AgCl~solution}{0.245~mol~Ag^+}\right) = ~?$$

This was how I worked it; perhaps you can see where you went wrong. I still can't see the problem exactly because you haven't shown the units on 0.01 mol.

Last edited: Oct 25, 2009