Answer
$M(vitamin B12)=1356.3\frac{g}{mol}$
Work Step by Step
$\omega (Co) = \frac{Ar(Co)}{M_{r}(vitamin B12)}\implies 0.0435=\frac{59}{M_{r}(vitamin B12)}\implies M(vitamin B12)\approx 1356.3\frac{g}{mol}$
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