Answer:
There are 4.5 x 10⁴⁶molecules of water in the world´s oceans
Step-by-step explanation:
The mass of the water molecule is the sum of the masses of each atom that composes the molecule:
The water molecule has 2 H (hydrogen) atoms and 1 O (oxygen) atom (H₂O₁ but the "1" in oxygen is never written, so you have H₂O).
The mass of a single H₂O molecule is then:
mass H₂O = 2(mass H) + mass O = 2 * 1.0 u + 16 u = 18u
The "u" stands for "atomic mass unit" and 1 u = 1.7 x 10⁻²⁷ Kg (rounded to two significant figures, since all numbers in the question have 2 significant figures: 1.0, 1.4 x 10²¹ and 16).
The mass of 1 water molecule in Kg is:
mass of H2O(kg) = 18 u * (1.7 x 10⁻²⁷ Kg/1.0 u) = 3.1 x 10⁻²⁶ kg
Now, we have the mass of a single water molecule and the mass of the world´s oceans:
3.1 x 10⁻²⁶ kg ⇒ 1 water molecule
1.4 x 10²¹ kg ⇒ x = 1.4 x 10²¹ kg * 1 molecule/ 3.1 x 10⁻²⁶ kg
= 4.5 x 10⁴⁶molecules of water