Counting of Atoms and Law of Conservation of Mass

Counting Atoms - Conservation of Mass

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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 (Na) 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

Which answer correctly lists the number of each atom in this compound?

Na₂S₂O₃

• 2 Sodium (Na)

  2 Sulfur (S)

  3 Oxygen (O)

• 1 Sodium (Na)

  2 Sulfur (S)

  2 Oxygen (O)

• 2 Sodium (Na)

  2 Sulfur (S)

  6 Oxygen (O)

• 1 Sodium (Na)

  1 Sulfur (S)

  1 Oxygen (O)

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

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

Coefficient

• 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

Distributing the Coefficient

• Multiply the coefficient with each subscript

• 3 x 2 = 6 Nitrogen atoms

• 3 x 4 = 12 Oxygen atoms

Multiple Choice

How many Hydrogen atoms are in 2H₂O?

• 2

• 4

• 6

• 1

Multiple Choice

How many Oxygen atoms are in 6Li₂O?

• 2

• 12

• 6

Multiple Choice

How many Carbon atoms are in 3CO₂?

• 3

• 6

• 2

Multiple Choice

Which answer correctly lists the number of each atom in this compound?

2AgNO₃

• 1 Silver (Ag)

  1 Nitrogen (N)

  3 Oxygen (O)

• 2 Silver (Ag)

  1 Nitrogen (N)

  3 Oxygen (O)

• 2 Silver (Ag)

  2 Nitrogen (N)

  3 Oxygen (O)

• 2 Silver (Ag)

  2 Nitrogen (N)

  6 Oxygen (O)

​Law of Conservation of Mass

Like energy, mass cannot be created or destroyed.

The amount of mass at the start = the amount of mass at the end

In a chemical reaction, the number of atoms of each element must be balanced.

If you start with 4 Hydrogens, you must end with 4 Hydrogens

​Notice there are equal numbers of Hydrogen atoms and Oxygen atoms on each side.

Balanced Equations

• Follow the Law of Conservation

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

• They just get rearranged!

​Unbalanced Balanced

Al + O₂ → Al₂O₃ 4Al + 3O₂ → 2 Al₂O₃
Products: Reactants: Products: Reactants:
Al: 1 Al: 2 Al: 4 Al: 4
O: 2 O: 3 O: 6 O: 6

Multiple Choice

Balanced or Unbalanced?

H₂ + Cl₂ → 2HCl

• Balanced

• Unbalanced

Multiple Choice

Balanced or Unbalanced?

Li + O₂ → Li₂O

• Balanced

• Unbalanced

Multiple Choice

Balanced or Unbalanced?

H₂ + O₂ → H₂O

• Balanced

• Unbalanced

Multiple Choice

Balanced or Unbalanced?

N₂ + 6H₂ → 2NH₆

• Balanced

• Unbalanced

Multiple Choice

Balanced or Unbalanced?

Ca + 2HCl → 2CaCl₂ + H

• Balanced

• Unbalanced

Balancing Equations

You balance equations by adding coefficients in front of the elements and compounds.

You CANNOT change the subscripts!

This equation is unbalanced











H: 2 H: 3
N: 2 N: 1

Start to balance it by adding a 2 in front of NH₃ then recount your atoms









H: 2 H: 3 6
N: 2 N: 1 2

Finish by adding a 3 in front of H₂ then recount your atoms








H: 2 6 H: 3 6
N: 2 N: 1 2

Multiple Choice

Choose the coefficients that will balance the equation:

__ H_{2 +__} O₂ → __ H₂O

• 1, 1, 1

• 1, 2, 1

• 2, 1, 2

• 3, 2, 1

Multiple Choice

Choose the coefficients that will balance the equation:

__ NaCl → __ Na + __ Cl₂

• 2, 2, 1

• 1, 1, 1

• 1, 2, 1

• 2, 2, 5

Multiple Choice

Choose the coefficients that will balance the equation:

__ LiF +__ Br₂ → __ LiBr + __ F₂

• 1, 1, 1, 1

• 1, 2, 4, 1

• 2, 2, 1, 1

• 2, 1, 2, 1

Multiple Choice

Choose the coefficient that will balance the chemical equation:

__ Zn + __ HCl → __ ZnCl₂ + __ H₂

• 1, 2, 1, 1

• 2, 1, 1, 1

• 1, 2, 3, 4

• 1, 2, 2, 2

Poll

How comfortable do you feel with counting atoms in a compound?

• Very comfortable

• Okay

• So-so

• I have no idea what is happening

Poll

How comfortable do you feel with identifying if an equation is balanced or unbalanced?

• Very comfortable

• Okay

• So-so

• I have no idea what is happening.

Poll

How comfortable do you feel with balancing chemical equations?

• Very comfortable

• Okay

• So-so

• I have no idea what is happening