Conservation of mass

Counting Atoms - Conservation of Mass

How to count atoms in a chemical formaula.

• You already know how to read SUBSCRIPTS

• A subscript tells us how many of each atom there are

Multiple Choice

How many Sodium atoms are there in NaCl?

• 1

• 2

• 3

• 4

Multiple Choice

How many Magnesium atoms are there in Mg₃N₂?

• 1

• 2

• 3

• 4

Multiple Choice

How many Hydrogen atoms are there in Pentane, C₅H₁₂?

• 5

• 12

• 1

• 2

We CANNOT change the subscript, it would become something totally different!

• Add a subscript of 2 after Oxygen and you make Hydrogen Peroxide.

Coefficiant

• The large number in FRONT of the symbol. You have to multiply this number by all of the subscripts.

• 4 Iron Atoms

• 6 Oxygen Atoms

Multiple Choice

How many Hydrogens are in 2H₂O?

• 2

• 4

• 6

• 1

Multiple Choice

How many Oxygens are in 6H₂O?

• 2

• 12

• 6

Multiple Choice

How many Carbons are in 3CO₂?

• 3

• 6

• 2

Balanced Equations

• Follow the Law of Conservation

• Have the same number and type of atoms on both sides

• They just get rearranged!

Multiple Choice

Balanced or Not?

H₂ + Cl₂ ---> 2HCl

• Balanced

• Unbalanced

H₂ + O₂ ---> H₂O

The hydrogen are balanced, but the oxygens are not.

You CAN change the coefficient.

You CANNOT change the subscript.

We put a two in front of the water and this balances the oxygen.

H₂ + O₂ ---> 2H₂O

But....

H₂ + O₂ ---> 2H₂O

Reactants: 2H, 2O Products: 4H, 2O

• This causes the hydrogen to become unbalanced.

• To fix this, we place a two in front of the hydrogen on the left side.

  2H₂ + O₂ ---> 2H₂O

2H₂ + O₂ ---> 2H₂O

• It's balanced now!

• 4 H and 2 O

Multiple Choice

Balanced or Not?

H₂ + O₂ ---> H₂O

• Balanced

• Not

Multiple Choice

Balanced or not?

N₂+6H₂ ----> 2NH₆

• Balanced

• Unbalanced

REMEMBER

# of each element in reactants

MUST EQUAL

# of each element in products

___SiO₂ + ___C → ___Si + ___CO

Balance this equation.

__AgNO₃ + __MgCl₂

→

__AgCl + __Mg(NO₃)₂

Balance this equation.

___H₂SO₄ + ___NaOH

→

___Na₂SO₄ + ___H₂O

Balance this equation.

__C₃H₈ + __O₂ → __CO₂ + __H₂O

Balance this equation.

__C₈H₁₈ + __O₂ → __CO₂ + __H₂O

Balance this equation.