How many moles and what is the mass?

How many moles is #3.02 * 10^25 # molecules of H2O? What is the mass of this number of molecules?

1 Answer
Matthew G.
May 18, 2018

50.2 mols of Water
904 grams of Water

Explanation:

First off, we need to know what a mol is. A mol is an extremely large number representing how many molecules are in a substance. One mole is equivalent to #6.02*10^23# molecules so to find the number of mols, divide the total number of molecules by the number of molecules in one mol:

#(3.02*10^25) / (6.02*10^23) = 50.2#

This mean there are 50.2 mols of water present.

Now we need to find the molar mass of water to calculate the mass of water. The formula for water is #H_2O# meaning there are two Hydrogen atoms and one Oxygen atom in one molecule of water. Using the periodic table, we can find the molar mass of Hydrogen and Oxygen from their atomic mass units.

Hydrogen: #1.01 (g)/(mol)#
Oxygen: #16.00 g/(mol)#
#2*(1.01) + 16.00 = 18.02 (g)/(mol)#

This means for every mol of water, it weighs 18.02 grams. Now we can calculate the mass

#18.02 (g)/(mol) * 50.2 mol = 904 g#

Now we know there is a total of 904 grams of water present.

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