Which of the following has the highest melting point? A fragment of DNA that has

[iffets12345]

Okay, so Cr is undergoing reduction right, as it goes from Cr⁵⁺ to Cr³⁺

Fe must therefore undergo oxidation.  And it can only go from +2 to +3 state, so initially it must be Iron (II) thus option C is eliminated.

Then find mol Cr₂O₇ = cv = 0.100 x 0.01278 = 0.001278 mol

To find mol of Fe, you do the half equation:

Fe²⁺ ---> Fe³⁺ + e^-

Then you multiply by 6, so the electrons cancel out in the full equation.

Ratio Cr₂O₇^2- : Fe²⁺ is 1:6

Therefore, multiply mol by 6.

n(Fe²⁺)= 0.001278 x 6 = 0.007668 mol

m(Fe²⁺)= 0.007668 x 55.9 = 0.43 g

Now you test out all the alternatives.

A - Mol Cl is twice the mol of Fe, so 0.007668 x 2 x 35.5 = 0.54
0.54 + 0.43 = 0.97 g - wrong

B - Mol SO₄ is the same as the mol of Fe, so 0.007668 x (32.1 +64) = 0.74
0.74 + 0.43 = 1.17 g - wrong

C - we eliminated before

D - Mol NO₃ is twice the mol of Fe, so 0.007668 x 2 x (14 + 48) = 0.95
0.95 + 0.43 = 1.38 g - RIGHT!

Absolute pain in the ass question for only one mark...

I agree I did it stonecold's way, up until the last bit where I just scrapped the masses and checked for 1:6 ratios.
But yes, it was damn long. Don't think u can solve for it on ur own without options... unless  kyzoo or martoman care to find one or mao

[Martoman]

Haven't done this exam yet :S planning too soon given i've heard its hard.

[iffets12345]

tricky i reckon.

[fady_22]

You could do question 4 with molar masses: find the mole of dichromate, multiply by six to find the mole of iron. Since you have the mass, you can find the molar mass of the salt.
Then, subtract the molar mass of iron to be left with the molar mass of the anion. You will find that it is equal to 2*NO3, making D the right answer.