How many molecules are there in a 42.3 g sample of water, H2O?

In order to figure out how many molecules of water are present in that  

48.90-g

sample, you first need to determine how many moles of water you have there.

As you know, a mole is simply a very large collection of molecules. In order to have one mole of something, you need to have exactly  

6.022

⋅

10

23

molecules of that something - this is known as Avogadro's number.

In order to get to moles, you must use water's molar mass. A substance's molar mass tells you the mass of one mole of molecules of said substance.

Water has a molar mass of  

18.015 g/mol

. This means that one mole of water molecules has a mass of  

18.015 g

.

So, to sum this up,  

6.022

⋅

10

23

molecules of water will amount to  

1 mole

of water, which in turn will have a mass of  

18.015 g

.

Explanation: