question from achiever test 1

[bxs023100]

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hey guys, i was wondering if anyone could help me figure this problem out. its from test 1, question 18 in gen. chem.

it says what percent of iron is present in ore if a 1.12 g sample requires 25 ml of .050 M KMnO4 to fully oxidize all of the dissolved Fe+?

8 H+ + 5 Fe^2+ + MnO4- --> 5 Fe^2+ + Mn^2+ + 4H2O



i don't get their math. thanks!!

 

[Bob Faraples]

I like to work a little differently than what I have seen taught by most profs, by setting up your units that you are looking for first and go from there - for example:

we are looking for (grams Fe)/(grams ore) times 100

So start with tha top unit - grams Fe. What do you know that is in grams of Fe? Molar mass! so write

58gramsFe/1 mole Fe

Then to cancel the 1 mole Fe you need something else... like moles of KMnO4! so you have

(58gramsFe/1mole Fe)*(5molesFe/1moleKMnO4)

Then how can you cancel the moles of KMnO4 off the bottom? by putting moles of KMnO4 on the top! And we know that we have .05M KMnO4

(58gramsFe/1mole Fe)*(5molesFe/1moleKMnO4)*(.05molesKMnO4/L solution)

Then what did they give us that will let us get the L of KMnO4 solution off the bottom? 25ml! - you have to convert between ml and L on this one

(58gramsFe/1mole Fe)*(5molesFe/1moleKMnO4)*(.05molesKMnO4/L solution)*(1 L solution/1000 ml)*(25Ml)

At this point all you are lacking is grams of ore (1.2) and the times 100, so add that on there.

(58gramsFe/1mole Fe)*(5molesFe/1moleKMnO4)*(.05molesKMnO4/L solution)*(1 L solution/1000 ml)*(25Ml/1.2)*100
This may not help at all as many professors teach another way than this, and it is hard to change a tiger's stripes. However, I recommend giving it a shot - start with the top unit you are looking for and plug-n-chug.

Hope this helped!

 

[Danny289]

when we learn the concepts in Gen & Org chem specially in Gen chem we can approach the problems correctly an efficiantly.
just i simpelize for you with one funny example: price for one pound apple is 89 cents if I want to buy 5 pounds apple how much I have to pay
solution: write down the relation yo have and solve it( in stiochemistry 99% same corelation between two substance exist .find those and write them down)
everything related to the apple under the apple and price under price.

apple ~ Price
1 ~ 89
5 ~ X ========> solving easy : X=(89x5)/1


exact same in G chem try to make it easy and understable to you ,
first: write what you have then finding to substance is easy

KMnO4 0.050 M (mol) & V= 25


Fe = 1.12 gr
second finding two substance are related now is easy just rember There is 5 Fe and 1 KMnO4

5 Fe ~ KMnO4

as i said simplize the problem for youself , for example if i have gr for Fe , if i have gr for Kmno4 i can go for gr ~gr
i can get gram for KMnO4:
is another apple and price!!

for KMnO4


Mol ~ Gr( mass)
1 ~ 158
0.05 ~ X ================> X= (158x0.05)/1 X= 7.9 gr

another apple and price !!between V and gram ( you must know the concept of molarity) Molar means gram in 1000 ml( lit) solution)

V ~ Gram
1000 ~ 7.9 ( cocepet of molarity M)
25 ~ X
============> find X ========.> X= (25x7.9)/1000
X= 0.1975 gr KMnO4

now we got the Gram for KMnO4 and the final Apple and price!!!!!! plus the concept of percent and yeild ( look at the kaplan !!)


5 Fe (gr ) ~ KMnO4 (gr)
5x 56 ~ 158
1.12 (X) ~ 0.1975 (X)= will be the answer , just multiply by 100


answer : 0.3125 ======> x 100 =====> 31%



grab the concepts

little quote: i tried to explain for mor clearification. imagine if you know concept you can do few of calculation in your mind , plus a little algebra will be very helpful and increase you speed for solving the problems . for example just write the answers as a ratio it will be simplize for example : 1.12 and 56 and also for kMnO4

i hoped it helped, appolgize for Apple and price !!!! and long explanation
if it make you confused, just forget what I've said, back to your method.

 

[laundry]

hey guys, i was wondering if anyone could help me figure this problem out. its from test 1, question 18 in gen. chem.

it says what percent of iron is present in ore if a 1.12 g sample requires 25 ml of .050 M KMnO4 to fully oxidize all of the dissolved Fe+?

8 H+ + 5 Fe^2+ + MnO4- --> 5 Fe^2+ + Mn^2+ + 4H2O


i don't get their math. thanks!!

Call me old-fashioned, but I love stoichiometry solutions. Check out the attached solution, I think that should be correct...

Cheers 👍

 

[littlemissy]

Call me old-fashioned, but I love stoichiometry solutions. Check out the attached solution, I think that should be correct...

Cheers 👍

thats the way i would have done it

 

[bxs023100]

thanks so much you guys!! you all explained it differently and that helped a lot. im usually pretty good at stoichiometry, but this one just threw me off.