Question

Chemical analysis shows that the electron transfer protein ferridoxin contains 3.72% Fe by mass. Calculate the minimum possible molar mass of ferridoxin. The actual molar mass is ca. 6,000 g/mol. Account for the discrepancy between your minimum value and the actual value.

Answer 1

Atomic mass of iron (Fe) = 56 g.mol^-1.

% mass of Fe in the given ferredoxin = 3.72

Let's consider no. of iron atoms in the ferredoxin = 1 (minimum no.)

i.e. 3.72 = (1 * 56 / molecular weight) * 100

The minimum possible molar mass of ferredoxin = (56/3.72) * 100, i.e. ~ 1500 g.mol^-1.

Since the minimum possible molar mass corresponds to a minimum no. of iron atoms (i.e. 1), the minimum value is (1/4)^th of the actual value (i.e. 6000 g. mol^-1, which indicates that the actual molecule contains four iron atoms).