Find the mass in grams of 1.55 x 1023 molecules of Cl2?

We are told that there are 1.55 x 10²³ molecules of Cl₂ and we need to calculate the mass of these molecules. We need to do several conversions. The easiest will be to convert the amount of molecules to the number of moles present. To do this, we need to use Avogadro's number which is 6.022 x 10²³ molecules/mole.

1.55 x 10²³ molecules / 6.022 x 10²³ molecules/mole = 0.257 moles Cl₂

Now that we have the moles of Cl₂ present, we can convert this value to a mass of Cl₂ by using the molecular mass of Cl₂. The molecular mass is 70.906 g/mol.

0.257 moles Cl₂ x 70.906 g/mol = 18.3 g Cl₂

Therefore, 1.55 x 10²³ molecules of Cl₂ will have a mass of 18.3 g.