- Determine the mass of 2.5 x 109 H2O molecules

- Determine the mass of 2.5 x 10⁹ H₂O molecules

One mole of H₂O is 6.022 x 10²³ molecules of H₂O (Avogadro's number). This relation is then used to 'convert' a number of H₂O molecules to grams by the ratio:

mass of X molecules of H₂O / X molecules = mass of a mole of H₂O molecules / 6.022 x 10²³ molecules

Solve for mass of X molecules of H₂O

mass of X molecules of H₂O = ( mass of a mole H₂O · X molecules of H₂O ) / 6.022 x 10²³ H₂O molecules

mass of 2.5 x 10⁹ molecules of H₂O = ( 18.02 g · 2.5 x 10⁹) / 6.022 x 10²³ H₂O molecules

mass of 2.5 x 10⁹ molecules of H₂O = ( 4.5 x 10¹⁰) / 6.022 x 10²³ H₂O molecules
mass of 2.5 x 10⁹ mass of 2.5 x 10⁹ molecules of H₂O = 7.5 x 10^-14 g.

Answer

The mass of 2.5 x 10⁹ molecules of H₂O is 7.5 x 10^-14 g.