[xxt]

Question:

If the known charge on the anion is double or triple the charge calculated by criss-crossing, then double or triple the charges for both anion and cation (list one example)

Example given: oxide ion = O2-  so, Pb4+ and O2- = lead (IV) oxide

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My understanding from reading the question:

Known charge on the anion = charge calculated (by criss-crossing) x2 (or x3)


My thoughts:  From the given example, I'm seeing that Pb4+ is the one whose charge is double or triple the charge calculated by criss-crossing, because Pb4+ and O2- = lead (IV) oxide = PbO2, so the charge of Pb4+ is double the charge of Pb, but it's a cation not an anion, this is why I'm confused as the example says " the known charge on the anion is double or triple the charge calculated by criss-crossing", am I not understanding the example correctly?

My understanding from reading the question:
Kown charge on the anion = charge calculated (by criss-crossing) x2 (or x3)

Please help me on this so I can move on, thank you guys very much!

[mjc123]

It applies equally to cations and anions. If you know the charge on one, you can work out the charge on the other. Thus for the formula PbO₂, the charges on Pb and O are in the ratio 2:1. But we know that oxide is O^2-, so Pb must be Pb⁴⁺.
And yes, you are misunderstanding the example, it says "the known charge on the anion..." So you know the charge on the anion, so you use that to work out the charge on the cation. Your equation would be better written as
Charge on cation = cation charge calculated by crisscrossing * known anion charge/anion charge calculated from crisscrossing
Of course you could do it the other way round, but more often it's the anion charge we know.