I need help in standard reduction potentials

What is meant by " (1 mol dm-3 with respect to both) "
in
https://www.thestudentroom.co.uk/attachment.php?d=1595612469&attachmentid=932612
I also do not understand the mark scheme to the following question - https://www.thestudentroom.co.uk/album.php?albumid=24266&attachmentid=932612

Mark scheme - https://www.thestudentroom.co.uk/album.php?albumid=24266&attachmentid=932614

My idea is that, since the solution should be 1moldm-3 with respect to both ions why not use 0.5 moldm-3 of both iron (II)sulphate and iron (III)sulphate?

I have doubt and I am weak in this topic.

And why use iron chloride instead of iron sulphate? ( according to mark scheme )

Help me!

Your first two links are to the same image.

In order to produce the standard emf for the Fe2+/3+ half-cell, you need both ions to be the same conc as each other. So you can use 0.5 mol dm-3 (as you suggest).

Since I can't see the Q, I can't comment about the Cl- vs SO42- part. My guess is that you're given data for the Cl2/Cl- half-cell and the emf of that half-cell will interfere. I know that Cl2 can oxidise Fe2+ to Fe3+ (or Cl- can reduce 3+ back to 2+).Sorry

Your first two links are to the same image.

In order to produce the standard emf for the Fe2+/3+ half-cell, you need both ions to be the same conc as each other. So you can use 0.5 mol dm-3 (as you suggest).

Since I can't see the Q, I can't comment about the Cl- vs SO42- part. My guess is that you're given data for the Cl2/Cl- half-cell and the emf of that half-cell will interfere. I know that Cl2 can oxidise Fe2+ to Fe3+ (or Cl- can reduce 3+ back to 2+).