gravimetric analysis

Question

Iron in an ore is gravimentrically analyse by weighing Fe₂O₃ precipitate. If the iron content in the range 10% to 14%, what is the minimum mass of sample (in four significant result) required to obtained 100.0 mg of precipitate?

Answer 1

atomic mass of Fe is 55.85 g/mol

molar mass of Fe₂O₃ is 159.69 g/mol

mass percentage of Fe in Fe₂O₃=55.85*2/159.69=69.95%

100mg Fe₂O₃ contains Fe 100*69.95%=69.95 mg

Fe occupies 10-14% of sample mass

sample mass in the range of 69.95/14%-69.95/10%, namely 499.6-699.5 mg

minimal sample mass is 499.6mg