Question

Pheromones are a special type of compound secreted by the females of many insect species to attract the males for mating. one pheromone has the molecular formula c19h38o. normally, the amount of this pheromone secreted by a female insect is about 1.8 10-12 g. how many molecules are there in this quantity?

Answer 1

Molar mass of C is 12.01g/mol.
Molar mass of H is 1.01g/mol.
Molar mass of O is 16.00g/mol.

If we have 19 atoms of C, 38 atoms of H, and 1 atom of O, then we have:
19*12.01+38*1.01+1*16.00=282.57g/mol
282.57g/mol is the molar mass of the pheromone molecule.

Supposing that "1.8 10-12 g" is 1.8*10^(-12)g then, this quantity of pheromones, shoud be divided by the molar mass of the pheromone molecule:
1.8*10^(-12)/282.57g/mol=6.37*10^(-15)moles

Using now Avogadro's number of molecules in 1mol (6.02*10^(23)) it is possible to know how many molecules are there in this pheromone quantity.
6.02*10^(23)/6.37*10^(-15)=9.45*10^(37)molecules

So, in this quantity of the pheromone there are approximately 9.45*10^(37)molecules of it.