How do you solve this physics question?

Question

One molecule of water (H2O) contains two atoms of hydrogen and one atom of oxygen. A hydrogen atom has a mass of 1.0 u and an atom of oxygen has a mass of 16 u, approximately.

(a) What is the mass in kilograms of one molecule of water?

(b) How many molecules of water are in the world's oceans, which have an estimated total mass of 1.4 × 10^21 kg?

Answer 1

Mass of H=1.0u             Mass of O=16u

A) There are 2 atoms of H and 1 atom of O so mass of one water molecule is 18.0u. We must convert it to kg.

1atomic mass unit (u)=1.66053886x10^-27kg   so...

(18.0u)(1.66053886x10^-27kg/1u) = 2.98896995x10^-26kg

B) Total mass=1.4x10^21kg  so...

1.4x10^21kg/2.98896995x10^-26kg = 4.683887839x10^46 molecules

Answer 2

We do know that 1 g-mole of water is 18 grams or 0.018 kg, and that 1 molecule is 1/A# the weight of one g-mole, where A# is Avagodro's Number.  So one molecule has the weight of  18x10^-3 kg/mole/6.02x10^23 molecules/mole or about 3x10^-26 kg.

From part a, you should be able to work your way through part b.  For starters, find the g-moles of water in the oceans.  

Answer 3

1 u = 1.660538782(83)×10−27  kg

Thats all you should really need