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a level ocr chemistry

help please!

In Stage 1, a student uses a 0.655 g sample of impure FeSO4.7H2O.
In Stage 3, the student uses 19.6 cm3 of 0.0180 mol dm–3 Cr2O72– to reach the end-point.
One mole of Cr2O72– reacts with 6 moles of Fe2+.
Calculate the percentage purity of the impure sample of FeSO4.7H2O.

answer: Moles of dichromate(VI) = 3.53 × 10–4 (1);
Moles of iron(II) = 2.12 × 10–3 (1):
Mass of hydrated iron(II) sulphate = 0.588 g (1);
Percentage purity = 89.8 / 89.8 – 90.0 (1)
i dont understand where they got the 0.558g from