N=mass /Mr = 9.794/392.16 = 0.0249745

C=m/vx1000 = 0.0249745 x1000 / 250 = 0.099898 therefore = 0.1

Please, be consistent. First, you calculated some N, then, in the next formula, you used some m. It is not easy to followe, you know.

At least your concentration is OK (almost - I would put it as 0.0999).

Now, if concentration of Fe(NH

_{4})

_{2}(SO

_{4})

_{2}.6H

_{2}O is 0.1, what is concentration of Fe

^{2+}? Before you will start to multiply/divide anything by anything, just think - how many moles Fe

^{2+} per mole of Fe(NH

_{4})

_{2}(SO

_{4})

_{2}.6H

_{2}O?