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# chemistry help watch

1. question:
a 250cm3 solution is prepared by dissolving 9.066g of Fe(NO3)2.xH20 in water. 25cm3 samples of this solution are acidified and titrated against 0.0200moldm-3 K2Cr2O7. the mean titre is 26.25cm3. Calcukate the molar mass, the value of x and the formula of the salt.

for the molar mass of the fe2+ sample, im getting 575.6 which is wrong but idk why.
help is appreicaed x
2. (Original post by penelopecrux)
question:
a 250cm3 solution is prepared by dissolving 9.066g of Fe(NO3)2.xH20 in water. 25cm3 samples of this solution are acidified and titrated against 0.0200moldm-3 K2Cr2O7. the mean titre is 26.25cm3. Calcukate the molar mass, the value of x and the formula of the salt.

for the molar mass of the fe2+ sample, im getting 575.6 which is wrong but idk why.
help is appreicaed x
How did you get 575.6? Show your working if you can.
3. (Original post by CheeseIsVeg)
How did you get 575.6? Show your working if you can.
so ive got the overall equation and it's as follows:
14H + Cr2O7 2- +3Fe2+ --> Cr3+ +7H20 + 3Fe3+

n(cr2o72-) =0.02 x 0.02625 = 5.25 x 10^-4 mols
thus, n(Fe2+) = 1.575 x 10^-3 (1:3 ratio)

then i found the number of moles in 0.25dm3 which i found to be 0.01575 mols.

then i did 9.066/0.01575 to give me 575.6 gmol-1 which is wrong, and idk why :/
4. (Original post by penelopecrux)
so ive got the overall equation and it's as follows:
14H+ + Cr2O7 2- +3Fe2+ --> Cr3+ +7H20 + 3Fe3+

n(cr2o72-) =0.02 x 0.02625 = 5.25 x 10^-4 mols
thus, n(Fe2+) = 1.575 x 10^-3 (1:3 ratio)

then i found the number of moles in 0.25dm3 which i found to be 0.01575 mols.

then i did 9.066/0.01575 to give me 575.6 gmol-1 which is wrong, and idk why :/
The charges don't balance.
The 14H you wrote should have been 14H+.
5. (Original post by BobbJo)
The charges don't balance.
The 14H you wrote should have been 14H+.
ok, thats not an issue tho, it doesnt change my answer
6. (Original post by penelopecrux)
ok, thats not an issue tho, it doesnt change my answer
It does if your atoms are not balanced either ...

look at the chromium
7. (Original post by charco)
It does if your atoms are not balanced either ...

look at the chromium
omg, im telling u this does not make a difference to the fact that im getting 575.6g mol-1!!
8. (Original post by penelopecrux)
omg, im telling u this does not make a difference to the fact that im getting 575.6g mol-1!!
If you do not balance the equation correctly you cannot possibly calculate the equivalent moles ...

omg

9. (Original post by penelopecrux)
ok, thats not an issue tho, it doesnt change my answer
It does. With your (unbalanced) equation you get the wrong answer. With the correct balanced equation you get the correct answer.
10. (Original post by charco)
If you do not balance the equation correctly you cannot possibly calculate the equivalent moles ...

omg

ok this is the equation im geting:

14H+ + Cr2O72- + 3Fe2+ --> Cr3++7H20 +3Fe3+

THis makes no difference to my answee at all!!
11. (Original post by penelopecrux)
ok this is the equation im geting:

14H+ + Cr2O72- + 3Fe2+ --> Cr3++7H20 +3Fe3+

THis makes no difference to my answee at all!!
Because it's wrong. The charges don't balance.
12. (Original post by BobbJo)
Because it's wrong. The charges don't balance.
what do u mean, why dont they balance??
13. (Original post by penelopecrux)
what do u mean, why dont they balance??
On the LHS the charges add up to +18
On the RHS the charges add up to +12
14. (Original post by BobbJo)
On the LHS the charges add up to +18
On the RHS the charges add up to +12
but i dont get why u included the stoichiometry.
15. (Original post by penelopecrux)
but i dont get why u included the stoichiometry.
What stoichiometry?

The chromium atoms are not balanced.

The charges are not balanced.
16. (Original post by penelopecrux)
ok this is the equation im geting:

14H+ + Cr2O72- + 3Fe2+ --> Cr3++7H20 +3Fe3+

THis makes no difference to my answee at all!!
Just out of curiosity, what do you think that the large numbers in front of the species (coefficients) actually MEAN.

What do the subscripts in the formulae actually MEAN.

"Balancing" requires that the number of atoms of ALL TYPES on either side of the reaction arrow are equal.

It also requires that the electronic charges are equal.

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