A student is given the assignment to determine the mass iron in a brand of women
ID: 917109 • Letter: A
Question
A student is given the assignment to determine the mass iron in a brand of women's vitamins. He begins by taking four vitamin tablets, grinding the tablets into a fine powder to undergo the necessary sample preparation to free all iron from the sample matrix and to reduce all iron to the +2 state. The resulting Fe^2+ solution is transferred to a 250 ml. volumetric flask and diluted to volume with distilled water. A 50 mL aliquot of the sample requires 42.98 mL of 1.00 x 10 3 M Cr,02'. Calculate the mg Fe per tablet. C r2 O^2- 7- + 6 Fe^2+ + 14H 2 Cr^3+ + 6 Fe^3+ + 7 H20Explanation / Answer
Number of moles of Cr2O72- is , n = Molarity x volueme in L
= 1.00x10-3M x 0.4298 L
= 4.298x10-4 mol
According to the balanced equation given above,
1 mole of Cr2O72- reacts with 6 moles of Fe2+
4.298x10-4 mol of Cr2O72- reacts with 6x 4.298x10-4 moles= 2.579x10-3 mol of Fe2+
This is the number of moles of in 50 mL of the aliquot.
For 250(5x50) mL of the aliquot the number of moles is = 5x 2.579x10-3 mol of Fe2+
= 0.0129 moles
This 250 mL solution is prepared by grinding 4 tablets
So for one tablet the number of moles = (0.0129)/4
= 3.22x10-3 moles
Molar mass of Fe is = 55.84 g/mol
So mass of Fe in each tablet = number of moles x molar mass
= 3.22x10-3 moles x 55.84 (g/mol)
= 0.18 g
= 0.18x1000 mg
= 180.0 mg
Therefore the mass of Fe in each tablet is 180.0 mg
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