# Molarity/redox reactions

## 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?

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

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

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

what i dont 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

Borek
Mentor
OK

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

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

what i dont 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 dont 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
Mentor
Ratio is given by reaction equation. Take a look at stoichiometric coeffcients.

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

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

what i dont 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
Mentor
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?