Water has a molecular mass of 18 amu. How many molecules of water are there in 18000 milligrams of water?

Question

Water has a molecular mass of 18 amu. How many molecules of water are there in 18000 milligrams of water?

  a, 12.04 X 10^23

 

b,  6.02 X 10^25

 

c,  18

 

 d,  6.02 X 10^23

 

e,  100

 

Answer 1

We first want to convert mg of water to g of water then to moles of water:

18,000 mg x 1 g / 1000 mg = 18 g water

18 g water x 1 mol/18 g = 1 mole water

Next we use Avogadro's number to find the number of molecules of water:

1 mol water x 6.02x10²³ molecules/mol = 6.02x10²³ molecules of water (answer d)