ERROR ANALYSIS
When doing a lab, you cannot know exactly what is happening, and there are sometimes errors that you cannot
avoid. In a lab, there are certain quantitative values that you get by direct measurement and other quantitative
values that you get indirectly by using calculations and making the assumption that the direct measurements were
made without error. Errors in a direct measurement trickle down toward calculated values, and then ultimately
toward the final calculation.
For each error, indicate:



The calculated number value that is affected by the error
Whether the calculate number is too high or too low, and an explanation for why
Whether your resulting calculation is too high or too low
EMPIRICAL FORMULA BY SYNTHESIS (Version #1)
In this lab, A (s) + B (g)  AxBy (s). Note that A is a solid, and when heated in air, B bonds to it to form the solid
compound AxBy. Your goal in this lab is to determine the empirical formula of AxBy (i.e. ratio of A:B). The mass of A
is directly measured, but the mass of B is determined indirectly by subtracting the mass of A +evaporating dish
before heating from the mass of AxBy + evaporating dish after heating.
Error
Indirect/
calculated
number
affected
Too high/ low?
WHY?
The evaporating dish was not
fumed dry so it was wet at the
beginning of the experiment.
Mass of B
Too low, because the excess water included in the mass
of A evaporates off (losing mass) rather than reacting
with B to turn into AxBy (gaining mass), contributing to a
smaller mass gain than would be expected.
The sample of A was handled
Mass of B
with wet hands so it was wet at
the beginning of the experiment.
The sample of A was already
partially reacted into AxBy before
the start of the experiment.
Mass of B
Some of the solid spattered out
during the reaction of A with B.
Mass of B
Some of the reacted AxBy
absorbed moisture from the air
while cooling.
Mass of B
A:B
ratio
too
high/
low?
Too
high (B
too low)
EMPIRICAL FORMULA BY DECOMPOSITION (Version #2)
In this lab, CxDy (s)  C (s) + D (g). Note that CxDy is a solid, and when heated in air, CxDy decomposes to form the
solid element C and gaseous element D. Your goal in this lab is to determine the empirical formula of CxDy (i.e. ratio
of C:D). The mass of C is directly measured, but the mass of D is determined indirectly by subtracting the mass of C
+evaporating dish after heating from the mass of CxDy + evaporating dish before heating.
Error
Indirect/
calculated
number
affected
The evaporating dish was not
Mass of D
fumed dry so it was wet at the
beginning of the experiment.
The sample of CxDy was handled Mass of D
with wet hands so it was wet at
the beginning of the experiment.
The sample of CxDy was already
partially decomposed into C and
D before the start of the
experiment.
Mass of D
Some of the solid spattered out
during the decomposition of
CxDy.
Mass of D
Some of the solid C absorbed
moisture from the air while
cooling.
Mass of D
Too high/ low?
WHY?
Too high, because the excess water included in the
mass of CxDy evaporates off (losing mass) contributing
to a bigger mass loss than would be expected.
C:D
ratio
too
high/
low?
Too low
(D too
big)
HYDRATE COMPOSITION
In this lab, ExFy z H2O (s)  ExFy (s) + z HxO (g). Note that ExFy z H2O is a solid, and when heated in air, the hydrate
ExFy z H2O decomposes to form the solid anhydrate ExFy and gaseous H2O. Your goal in this lab is to determine the
percentage of water in this hydrate. The mass of ExFy is directly measured, but the mass of H2O is determined
indirectly by subtracting the mass of ExFy +evaporating dish after heating from the mass of ExFy z H2O + evaporating
dish before heating.
Error
Indirect/
calculated
number
affected
Too high/ low?
WHY?
% H2O
too
high/
low?
The evaporating dish was not
fumed dry so it was wet at the
beginning of the experiment.
The sample of ExFy z H2O was
handled with wet hands so it
was wet at the beginning of the
experiment.
Mass of
H2O
Too high, because the excess water included in the
mass of CxDy evaporates off (losing mass) contributing
to a bigger mass loss than would be expected.
Too
high
The sample of ExFy z H2O was
already partially decomposed
into ExFy and HxO before the
start of the experiment.
Mass of
H2O
Some of the solid spattered out
during the decomposition of
ExFy z H2O.
Mass of
H2O
Some of the solid ExFy absorbed
moisture from the air while
cooling.
Mass of
H2O
Mass of
H2O