Question

determine (1) % Fe in unknown sample

(2)average % Fe

(3)standard deviation

(4)relative standard deviation

For gravimetric determination of Iron as Fe2O3, how do I calculate?

weighed sample of unknown containing Fe to produce ~0.3g of Fe2O3 (~0.6 - 1 g of unkonwn)

I got 0.75g from unknow sample.

I got 0.05g from Fe2O3 collected from unknow.

Answer 1

Fe₂O₃2Fe + (3/2)O₂ Fe₂O₃

i.e. 2 moles of Fe on oxidation gives 1 mole of Fe₂O₃.

Total weight of unknown = 0.6 - 1 g, i.e. let's consider an average of 0.8 g

(1) 0.3 g. of Fe₂O₃ = 0.3 g / 159.687 g mol^-1 = 0.00188 mol

i.e. The no. of moles of Fe = 2*0.00188 = 0.00376 mol = 0.00376 mol * 55.845 g mol^-1 = 0.21 g

The %Fe = (0.21/0.8) * 100 ~ 26.23% Fe

0.75 g. of Fe₂O₃ = (0.75/0.3)*26.23 = 65.57% Fe

0.05 g. of Fe₂O₃ = (0.05/0.3)*26.23 = 4.37% Fe

(2) The average % Fe (x̅​) = (26.23 + 65.57 + 4.37) / 3 = 32.057%

(3) The standard deviation (s) = [{(26.23-32.057)² + (65.57 - 32.057)² + (4.37 - 32.057)²} / (3 - 1)]^1/2 = 31.013%

(4) The relative standard deviation = (s/x̅​)*100 = (31.013/32.057)*100 = 96.74%