Law of Conservation of Mass
Law of Conservation of Mass:
 Mass is neither created nor destroyed during a
chemical reaction- it is conserved
 Mass reactants = Mass products
Chemical Equations/ Law of
Conservation of Mass
 For an equation to follow the Law of Conservation of
Mass, the equation must be balanced.
 Atoms can neither be created nor destroyed in an
ordinary chemical reaction

There must be the same number of atoms on both sides of
the equation
Parts of a Chemical Equation:
 Chemical equations tell us what substances are
reacting and what substances are being produced.

Reactants  Products
• Reactants- starting substances
• H2 + O2 →
• Products- substances formed during the reaction.
•
→ H2 O
Numbers found in a chemical reaction
 Subscripts The small numbers to the lower right of chemical symbols

Represent the number of atoms of each element in the molecule
 Example 1: How many atoms of each element are in one
formula unit of Calcium Nitrate: Ca(NO3)2 ?
Ca= 1
N= 2
O=6
 Example 2: How many atoms of each element are in one
formula unit of Magnesium Oxide: MgO ?
Mg= 1
O= 1
Numbers found in a chemical reaction
 Coefficients
 The large numbers in front of chemical formulas

Coefficients represent the number of molecules of the substance in the
reaction.

Example 1: How many atoms of each element are there in 3 formula units
of Calcium Nitrate: 3Ca(NO3)2 ?


Ca= 3
N= 6
O= 18
Example 2: How many atoms of each element are there in 2 formula units
of Magnesium Oxide:
2MgO
Mg = 2
O=2
Writing Chemical Equations
 Check for diatomic molecules:
 H2 , N2 , O2 , Cl2 , F2 , Br2 , I2

If these elements appear by themselves in an equation, they must be
with the subscript 2.
How do we balance the chemical
equation:
 Balance equations by changing coefficients
 Never by changing subscripts
How do we balance the chemical
equation:
 Steps:
 Write all reactants on the left and all products on the right side of the equation
arrow. Make sure you write the correct formula for each element.
 Use coefficients in front of each formula to balance the number of atoms on
each side.
 Multiply the coefficient of each element by the subscript of the element to count
the atoms. Then list the number of atoms of each element on each side.
 It is often easiest to start balancing with an element that appears only once on
each side of the arrow. These elements must have the same coefficient. Next
balance elements that appear only once on each side but have different numbers
of atoms. Finally balance elements that are in two formulas in the same side.
Time to Practice!!
Practice Problems:
Balance these equations:
 _____Zn + ______HCl  _____ZnCl2 + _____H2
 _____KClO3  _____KCl + _____O2
 _____S8 + _____F2  _____SF6
 _____Fe + _____O2  _____Fe2O3