How many molecules are contained in 16.0 g of benzene?

One mole of a substance contains an Avogadro's number of molecules, which is equal to `6.023 xx 10^23` molecules. 

One mole of a substance contain one molar mass of that particular substance. The molar mass of benzene (chemical formula: `C_6H_6`) is 78 g/mole ( = 6 x 12 + 6 x 1).

In other words, 78 grams of benzene is present in 1 mole of benzene.

This is also the same as saying 78 g of benzene contain 6.023 x 10^23 molecules.

Here, we have only 16 g of benzene. Therefore, by unitary method:

1 g of benzene contains (6.023 x 10^23)/78 molecules

and 16 g benzene contains 16 x (6.023 x 10^23)/78 = 1.24 x 10^23 molecules of benzene.

The same can also be calculated by knowing that 16 g of benzene is equivalent to 16/78 = 0.205 moles of benzene and then multiplying it by Avogadro's number to determine the number of molecules.

Hope this helps.