Name ___________________________________________
Hour _______________
Law of Conservation of Mass
Law of conservation of mass: the mass of all substances that are present before a
chemical change equals the mass of all the substances that remain after the change.
Total mass before change = Total mass after change
Lets look at the chemical change of hydrogen (H2) reacting with Chlorine (Cl2) to create
hydrochloric acid (HCl).
Here is an unbalanced equation of the reaction:
Reactant(s) H2  Cl2 HCl Product(s)
There has to be the same amount of atoms for each element on both sides of the equation.
On the left side we have 2 Hydrogens and Chlorines, and on the right side we have 1 of
 of mass tells us they need to equal on both sides. We place
each. The law of conservation
a 2 in front of HCl, like so 2HCl, and we would have a balanced equation.
Another way to look at it is with the atomic masses. We’ll add up the atomic masses on
both sides and see how they compare, and determine if we need to make any changes.
Reactants:
We see that we have 2 Hydrogens. The atomic mass of Hydrogen is 1.008. Since we have
two of them we need to multiply to find the total atomic mass for the molecule H2.
H2= 1.008 x 2 = 2.016 da (Daltons: a unit used for atomic mass)
Cl2= 35.453 x 2 = 70.906 da
Total Reactants’ Mass= 72.922 da
Products:
We see that we have 1 Hydrogen and 1 chlorine, so we can just add up their atomic mass.
HCl= 1.008 + 35.453 = 36.461 da
Total Product’s Mass= 36.461 da
1. What do you notice about the total Product’s mass compared to the total reactants’
mass?
Name ___________________________________________
Hour _______________
2. What would we need to do in order to have the mass of the products equal the mass of
the reactants?
Here is the balanced equation of the reaction:
H2  Cl2 2HCl
We now have the same amount of hydrogen and chlorine molecules on both sides of the
chemical equation.

3. If 50 g of sodium reacts with 25 g of chlorine, how many grams of sodium chloride
(NaCl) should we have at the end of the reaction?
4. If 50 grams of sodium reacts with chlorine to form 126 grams of salt. How many
grams of chlorine reacted?
5. If 20 grams of aluminum reacts with 200 grams of bromide to form aluminum
bromide, and no aluminum is left after the reaction, but 23 grams of bromine remained
unreacted. How any grams of aluminum bromide were formed?
6. A 10 gram sample of iron reacts with oxygen to form 18.2 grams of ferric oxide. How
many grams of oxygen reacted?