Calculating Molarity in Redox Reactions: NaCl Solution Example

[pmart491]

Homework Statement

What is the molarity of a NaCl solution if 18.3mL of the solution reacted with 13.6mL of 0.1M KMnO4 based on the following unbalanced redox reaction in an acidic solution?

Cl- + MnO4- yields-> Cl2 + Mn2+

The Attempt at a Solution

I did the two half reactions
Cl- -> Cl2
MnO4- -> Mn2+

Do I find the full half reactions (ie: the electrons gained/lost H2O, H+) and then do a molarity problem?

 

[Borek]

Yes - this is in fact simple stoichiometric question, just based on the redox reaction. Find full balanced reaction equation first.

 

[pmart491]

Ok I got 10Cl- + 16H+ + 2MnO4- -> 5Cl2 + 2Mn2+ + 8H2O

Next I did 13.6mL KMnO4 x .1M = .00136moles KMnO4

what i don't understand is how to find moles of NaCl through the half reaction

 

[pmart491]

well i think i got it

based on the half reaction, there are 5 times as many Cl- ions as there are MnO4- ions.
therefore there are 5 times as many moles of NaCl than there are moles of KMnO4.
5 x .00136=.0068moles NaCl

.0068moles NaCl/.0186L NaCl=.372M NaCl

 

[Borek]

OK

 

[Asian Girl]

Ok I got 10Cl- + 16H+ + 2MnO4- -> 5Cl2 + 2Mn2+ + 8H2O

Next I did 13.6mL KMnO4 x .1M = .00136moles KMnO4

what i don't understand is how to find moles of NaCl through the half reaction

well i think i got it

based on the half reaction, there are 5 times as many Cl- ions as there are MnO4- ions.
therefore there are 5 times as many moles of NaCl than there are moles of KMnO4.
5 x .00136=.0068moles NaCl

.0068moles NaCl/.0186L NaCl=.372M NaCl

I don't understand this. If we have V of NaCl is 18.3 mL, can we find M by using M=mole/V. So the ratio of NaCl and KMnO4 isn't 1:1 ?

 

[Borek]

Ratio is given by reaction equation. Take a look at stoichiometric coeffcients.

 

[Asian Girl]

Ok I got 10Cl- + 16H+ + 2MnO4- -> 5Cl2 + 2Mn2+ + 8H2O

Next I did 13.6mL KMnO4 x .1M = .00136moles KMnO4

what i don't understand is how to find moles of NaCl through the half reaction

Ratio is given by reaction equation. Take a look at stoichiometric coeffcients.

Why do we have to do 10Cl + 16H ...?

Sorry, I just don't understand your step.

 

[Borek]

Because these coefficients where necessary to balance the equation. Do you know what it means to balance the equation and why the reaction equations needs to be balanced?