How Many Molecules Can Stand on the Head of a Pin

Theologians used to speculate about how many angels might stand on the head of a pin. However, since few people had ever seen an angel, it was difficult to resolve the question. Few people have ever seen a single molecule either. Can we count them The following worked examples should remind you how we do this. 

Q How many molecules are there in 1 cm of a 1 mg cm solution of macromolecules of relative molecular mass 50,000  

Q How many molecules might be found in 1 cubic micron (1 pm ) of such a solution  

Q Imagine a typical small bacterial cell of diameter around 1 micron (1 pm). What would be the effective molar concentration of a particular compound if there were just one such molecule per cell  

A Approximate volume of the cell (since we do not know its exact shape) will be of order (diameter) = lxl0 m = lxl0 dm (1 m =1000dm ). 

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