Molecules of SiO2 would have a mass of 12.40 g

1.244x10^23 molecules

Explanation:

From Avogadro's hypothesis, 1mole of any substance contains 6.02x10^23 molecules.

This means that 1mole of SiO2 contains 6.02x10^23 molecules

1mole of SiO2 = 28 + (2x16) = 28 + 32 = 60g

If 1 mole (i. e 60) of SiO2 contains 6.02x10^23 molecules,

Therefore, 12.40g of SiO2 will contain = (12.40 x 6.02x10^23) / 60 = 1.244x10^23 molecules