Percentage calculation of chromium compound

[Paulo2014]

Homework Statement

20g of Cr2(SO4)3 was used to make up an 800 ml batch of tanning liquor. When the tanning process was complete, a 10ml sample was taken and the chromium remaining in the sample was determined.
The chromium in the sample was oxidised to dichromate and then titrated with a 0.100molL solution of iron 2 ions.
Given that 16.5ml of the iron solution was needed, determine the percentage of the chromium used up in the tanning process

Homework Equations

The Attempt at a Solution

I am unsure as to how to get an answer, I get the number of moles of Cr3+ used in tanning but I don't know how to get a percentage of chromium.

 

[Borek]

What is percentage definition?

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