Question

A 0.198-g sample of parsley is pyrolyzed and diluted with 8.00 mL of 1.0 M HCl. A 0.1-g crystal of NaSCN is added to the solution. The concentration of the prepared solution is determined to be 0.0965 mM in [FeSCN]²⁺. Based on this result, what is the mass of Fe in a 100-g sample of parsley?

the answer is 21.8 mg Fe but i dont know why

Answer 1

It is given that,

Concentration of [FeSCN2]+ = 0.0965 mM = 9.65 x 10^-5 M

Volume of solution = 8 mL = 8 x 10^-3 L

=> Moles of [FeSCN]2+ = 9.65 x 10^-5 x 8 x 10^-3 = 7.72 x 10^-7 moles

1 mole of [FeSCN]2+ contains 1 mole of Fe3+

=> Moles of Fe3+ = 7.72 x 10^-7

=> Mass of Fe3+ = 7.72 x 10^-7 x 55.845 = 4.31 x 10^-5 g

0.198 g of sample contains 4.31 x 10^-5 g of Fe

=> 1 g sample contains (4.31 x 10^-5 / 0.198) = 2.18 x 10^-4 g

=> 100 g sample contains = 2.18 x 10^-4 x 100 = 0.0218 g = 21.8 mg Fe