Calculate the number of molecules that 27 g of H₂O contains.

To calculate the number of molecules in 27 g of H₂O, we first need to determine the number of moles of H₂O in 27 g.

The molar mass of H₂O is 18 g/mol (2(1.008 g/mol) + 15.999 g/mol).

Number of moles = Mass / Molar mass
Number of moles = 27 g / 18 g/mol
Number of moles = 1.5 mol

Now, we can use Avogadro's number to calculate the number of molecules in 1.5 mol of H₂O.

1 mol of any substance contains 6.022 x 10^23 molecules.

Number of molecules = Number of moles x Avogadro's number
Number of molecules = 1.5 mol x 6.022 x 10^23 molecules/mol
Number of molecules = 9.033 x 10^23 molecules

Therefore, 27 g of H₂O contains approximately 9.033 x 10^23 molecules.