LAW OF
CONSERVATION
OF MASS
Anatomy of a Chemical Equation
Ingredients are
called “reactants”
Results are
called “products”
2H2 + O2
2H2O
coefficient- how
many there are of
that molecule
reacts
with
“yields” or
produces
subscript- number
of atoms present
per molecule
LEARNING CHECK
How many atoms of each element are in the
following molecules?
1.
2.
3.
4.
H2SO4
Ca(OH)2
NaCl
3H2O
LAW OF CONSERVATION OF MASS




Mass cannot be created nor destroyed in a
chemical reaction.
Mass can be converted from one form to another
# of atoms in the reactants = # of atoms in the
products
mass of reactants = mass of products
2H2 + O2
2H2O
VISUALIZING THE LAW OF CONSERVATION OF MASS
Reactants
mass of reactants
Products
=
mass of products
In the procedure shown above, a calcium chloride solution is
mixed with a sodium sulfate solution to create the products
shown. Which of the following is illustrated by this activity?
A. the law of conservation of mass
B. the theory of thermal equilibrium
C. the law of conservation of momentum
D. the theory of covalent bonding
TEST QUESTION
Which of the following equations correctly shows
the conservation of mass?
a. HgO ---> Hg + O2
b. BaSO4 + C ---> BaS + CO
c. H2SO4 + 2KOH ---> K2SO4 + 2H2O
d. NiCl2 + 2NaOH ---> Ni(OH)2 + NaCl
According to the law of conservation of mass, how
much zinc was present in the zinc carbonate?
a. 40 g
b. 88 g
c. 104 g
d. 256 g
Accuracy and Precision
Accuracy
Accuracy is how close a
measured value is to
the actual (true) value.
The bullet holes on this
target are accurate but
not precise.
Precision
Precision is how close
the measured values
are to each other.
The bullet holes are
precise but not
accurate.
Examples
Practice
Look at each target.
Determine if the bullet
shots are precise,
accurate, both or
neither.
Significant Figures
Why are significant figures important?
 Scientists indicate the precision of measurements by the
number of digits they report.

A value of 3.92g is more precise than a value of 3.9g. The digits that
are reported are called significant figures.
 Significant figures (sig figs) include all known digits plus
one estimated digit.

So in the example above: 3.92 there are three sig figs—two known
and one estimated.
Rules for recognizing significant figures
 1. Non-zero numbers are always significant.




sig figs
2. Zeros between non-zero numbers are
always significant.
3. All final zeros to the right of the decimal
place are significant.
4. Zeros that act as placeholders are not
significant. (In this case, convert to
scientific notation to remove the
placeholder zeros.)
5. Counting numbers and defined
constants have an infinite number
of significant figures.
72.3 = 3
60.5 = 3 sig figs
6.20 = 3 sig figs
0.0253 = 3
4320 = 3
6 molecules
60sec = 1 min.
Let’s give it a try!
Determine the number of significant figures in the
following masses:
a. 0.00040230g
a. 405000kg
a. 508.0L
a. 3.1587 x 10-8g
How’d you do?
a. 0.00040230g
five significant figures
a. 405000kg
three sig figs
a. 508.0L
four sig figs
a. 3.1587 x 10-8g
five sig figs
Rounding for significant figures
 If the digit to the right of the last sig. fig. is <5, do not
change
2.532  2.53
 If the digit to the right of the last sig. fig is >5, round
up
2.536  2.54
 If the digit to the right of the last sig. fig. is = 5 and
followed by a number larger than zero, round up
2.5351  2.54