[nikita]

A certain amount of hydrogen peroxide was dissolved in 100. mL of water and then titrated with 1.68 M  KMnO₄ . How much H₂O₂was dissolved if the titration required 19.8 mL of the KMnO₄ solution?

2KMnO₄(aq)+H₂O₂(aq)+3H₂SO₄ (aq)  O₂(g)+2MnSO₄(aq)+K₂SO₄+4H₂O

heres what i did and i am coming up with the wrong answer.


100mL is .100 L. so

.0166mol H₂0₂/.100 = .166M H₂O₂

Answer call for grams, so i
.166M H₂0₂ x 34.016g = 5.65g H₂O₂

i tried the answer in various ways, but it keeps telling me to check my mL to L conversion, but i know theyre all right.  What am i doing wrong?

[nikita]

am i just not thinking about this correctly?  am i supposed to put .565 g for some reason?  if the program is telling me my mL to L conversion is off, then my answer must be wrong by a /1000 or *1000

if i leave the mols at .0166 and multiply by the molar mass, then i get .565 g.  if that is correct, i still dont understand why.

[Borek]

Your reaction is wrong.

http://www.chembuddy.com/?left=balancing-stoichiometry&right=balancing-failure

[nikita]

oooooh, in terms of balancing after the electrons.  ugh, once again i am an idiot.  its called redox titration and i didnt do any redox balancing here at all.  heh, thanks!

[nikita]

can i ask another assenine question?  if i have MnSO₄, rather than setting the whole compound to 0, I take SO₄ by itself and Mn by itself to get a charge of -2 for the SO₄ and 2 for the Mn?  and the same for K₂SO₄?  i had been setting this all equal to 0, unless they were free floating polyatomic ions. 

maybe i can make myself more clear.

like in SO₄, i would set the whole thing equal to -2 and get -2 for oxygen and 6 for Sulfur. 

but when i do MnSO₄ am i supposed to set the whole thing equal to 0, and take SO₄ all together as a -2 charge, thereby making Mn a +2 charge?  or should i post this in a new topic.  im posting it here because im still trying to balance that equation.

[Borek]

but when i do MnSO₄ am i supposed to set the whole thing equal to 0, and take SO₄ all together as a -2 charge, thereby making Mn a +2 charge?  or should i post this in a new topic.  im posting it here because im still trying to balance that equation.

Molecule is neutral, ions are not.

http://www.chembuddy.com/?left=balancing-stoichiometry&right=oxidation-numbers-method

[nikita]

thank you for taking the time to answer my questions and giving me links to info!