Equilibria, Energetics and Elements (F325) - June 2011 Exam.

yeah I wanna know that too I divided by 2 so scored only 1/4.

yes thank you...im so confused on this too.

C1: 99
C2: 100
S1: 98
Total for AS: 297/300

C3: 79
C4: Next week
M1: 82
Total needed for A = 480 - 297 - 79 - 82
= 22 !!! :-)

Listen, to the people who ask why you divide by 4 to get the DOC question on the jan. 11 paper:

you work out the moles of thiosulfate ions they give you (conc. x volume) - those figures are given. Then you see that it's two moles thiosulfate to two moles of Iodide ions so their molar quantities are equal (this is looking at equation 3)

Now look at equation 2: moles of Iodide ions are in equal ratio to Mn 3+ so moles of thiosulfate = moles of Mn 3+

Look at equation 1: It's 4 moles of Mn 3+ to one mole 02 so divide that figure by 4

Hope that helped you!

Listen, to the people who ask why you divide by 4 to get the DOC question on the jan. 11 paper:

you work out the moles of thiosulfate ions they give you (conc. x volume) - those figures are given. Then you see that it's two moles thiosulfate to two moles of Iodide ions so their molar quantities are equal (this is looking at equation 3)

Now look at equation 2: moles of Iodide ions are in equal ratio to Mn 3+ so moles of thiosulfate = moles of Mn 3+

Look at equation 1: It's 4 moles of Mn 3+ to one mole 02 so divide that figure by 4

Hope that helped you!

Why is oxygen divided by four? What equation are they getting it from?

Q7 a

Fe has oxidation state 6+ in FeO4 (2-) and oxidation state 3+ in Fe3+, so has changed by -3

O has oxidation state 2- in FeO4(2-) and oxidation state 0 in O2, so the change is +3, however, 2 oxygen atoms have been used to form O2, so the overall change is +4

to balance the lowest common factor of -3 and +4 is 12, so you multiplu FeO4(2-) by 4 to get:

4FeO4(2-) + n[H+] --> 4Fe(3+) + H2O + 02

now look at the overall molecular charges, as they have to balance on both sides:

we have (4x-2) + n(1) --> (4x3+) + 0 + 0

which is -8 + n1+ --> 12+
we need to get 12+ on both sides, so we need to add 20H+ ions to the left to get:

4FeO4(2-) + 20[H+] --> 4Fe(3+) + H2O + 02

simple balancing from here, 20H+ on right, so you need 10H2O:

4FeO4(2-) + 20[H+] --> 4Fe(3+) + 10H2O + 02

16 oxygens on left, 10 on the right, so we need 6 more, so times O2 by 3

4FeO4(2-) + 20[H+] --> 4Fe(3+) + H2O + 302

there's your answer

Listen, to the people who ask why you divide by 4 to get the DOC question on the jan. 11 paper:

you work out the moles of thiosulfate ions they give you (conc. x volume) - those figures are given. Then you see that it's two moles thiosulfate to two moles of Iodide ions so their molar quantities are equal (this is looking at equation 3)

Now look at equation 2: moles of Iodide ions are in equal ratio to Mn 3+ so moles of thiosulfate = moles of Mn 3+

Look at equation 1: It's 4 moles of Mn 3+ to one mole 02 so divide that figure by 4

Hope that helped you!

but in the 2nd equation its 2Mn(OH)3 and in the 1st equation its 4Mn(OH)3 so how can you say the molar rations are equal??? Is it not 1:2

so wudnt you doulbe the moles of 2Mn(OH)3 to get molesof 4Mn(OH)3 and then divide that by 4 to get moles of O2????

but in the 2nd equation its 2Mn(OH)3 and in the 1st equation its 4Mn(OH)3 so how can you say the molar rations are equal??? Is it not 1:2

so wudnt you doulbe the moles of 2Mn(OH)3 to get molesof 4Mn(OH)3 and then divide that by 4 to get moles of O2????

Someone tell me how to remember which enthalpy change is +ve and -ve!!

Major rep!

Someone tell me how to remember which enthalpy change is +ve and -ve!!

Major rep!

It's like this: 2 moles of Mn 3+ to everything else in equation 2.
4 moles of Mn 3+ to everything else in equation 1.
Equations 1 and 2 are completely independent of each other, so you can't compare the ratio of moles of Mn 3+ between them

Hope that helped