1. (2.3)Conservation of Mass
(p )

2. Law of Conservation of Mass
During a chemical reaction, the total mass and number of atoms of the reactants equals the total mass and number of atoms of the products.
Notice there are equal numbers of hydrogen atoms and oxygen atoms on both sides.

3. Law of Conservation of Mass
Recall for a chemical equation:
Reactant(s)  Product(s)
Law of conservation of mass says that….
mass reactants = mass products

4. Law of Conservation of Mass-Example (Figure 2.20 p148)
Let’s count the atoms
Reactants
Type of Atom
Products Na H O

5. Law of Conservation of Mass-Example (Figure 2.20 p148)
Let’s count the atoms
Reactants
Type of Atom
Products 2 Na 4 H O

6. Law of Conservation of Mass-Example (Figure 2.20 p148)
Let’s count the atoms
Reactants
Type of Atom
Products 2 Na 4 H O

7. Law of Conservation of Mass-Example (Figure 2.20 p148)
Let’s count the atoms
This is a balanced equation!
Reactants
Type of Atom
Products 2 Na 4 H O

8. How To Count Atoms
The symbol of an element represents one atom of that element.
Eg. Ca = one calcium atom

9. Counting Atoms
A subscript is a number written at the lower right corner behind the symbol of an element. If there are more than one atoms of the element in the molecule, then a subscript is used to indicate the number of atoms.
N2 = 2 atoms of nitrogen
CO2 = 1 atom of carbon & 2 atoms of oxygen

10. Counting Atoms
A subscript outside a bracket multiplies all the elements inside the bracket
Ba3(PO4)2
3 atoms of barium
1 x 2 atoms of phosphorous = 2
4 x 2 atoms of oxygen = 8
TOTAL atoms = 13

11. Counting Atoms
A coefficient is a number written in front of a chemical symbol and indicates the number of atoms of the element.
3 C = 3 carbon atoms
3 O2 = 3 molecules of O2 or 6 oxygen atoms

12. COUNTING ATOMS
A coefficient is a number written in front of a chemical formula and indicates the number of molecules of that compound.
2 H2O = 2 molecules of water
So H2O + H2O = 4 hydrogen atoms & 2 oxygen atoms

13. Classwork Worksheet 2.3 (C)
Let’s practice examples 1-3 together and then you can complete page 2