Question

analytical chemists can detect very small amounts of amoin acids , down to 3x10^-21 mol. how many molecules of an amino acid (mr=200) would this be ?

Answer 1

The number of particles in one mole is given be Avagadro's number 6.022×10^23

Multiply by number of moles.

3 ×10^-21 mol * 6.022 ×10^23 molecules/mol = 1,807 molecules
(rounded to nearest whole number)