CHEMISTRY*11* UNCERTAINTY* UNCERTAINTY*INMEASUREMENTS*
CHEMISTRY
11
UNCERTAINTY
There
is
no
such
thing
as
a
perfect
measurement.
Each
measurement
contains
a
degree
of
uncertainty
due
to
the
limits
of
instruments
and
the
people
using
them.
In
laboratory
exercises,
students
are
expected
to
follow
the
same
procedure
that
scientists
follow
when
they
make
measurements.
Each
measurement
should
be
reported
with
some
digits
that
are
certain
plus
one
digit
with
a
value
that
has
been
estimated.
UNCERTAINTY
IN
MEASUREMENTS
eg.
ruler
graduated
cylinder
beaker
scientific
balance
electronic
balance
The
uncertainty
is
usually
half
of
the
marking
on
the
glassware
or
equipment
for
eg.
a
grad
cylinder
marked
every
1
ml
would
have
an
uncertainty
of
0.5
ml
On
a
digital
instrument,
the
uncertainty
or
deviation
or
error
is
usually
listed
on
the
instrument.
TWO
MAIN
CAUSES
OF
UNCERTAINTY
IN
THE
LAB:
1.
human
error
(in
reading
the
measuring
tool)
2.
instrument
uncertainty
(inherent
in
its
operation)
HOW
UNCERTAINTY
RELATES
TO
YOUR
CHEMISTRY
LABWORK:
1.
Lab
data
contains
varying
degrees
of
uncertainty
2.
Graphing
points
includes
some
uncertainty
as
to
the
placement
of
coordinates
3.
Calculations
compound
uncertainty
in
the
various
values
used
in
a
calculation
Two
concepts
that
have
to
do
with
measurements
are
accuracy
and
precision:
ACCURACY
The
accuracy
of
the
measurement
refers
to
how
close
the
measured
value
is
to
the
true
or
accepted
value.
For
example,
if
you
used
a
balance
to
find
the
mass
of
a
known
standard
100.00
g
mass,
and
you
got
a
reading
of
78.55
g,
your
measurement
would
not
be
very
accurate.
One
important
distinction
between
accuracy
and
precision
is
that
accuracy
can
be
determined
by
only
one
measurement,
while
precision
can
only
be
determined
with
multiple
measurements.
PRECISION
Precision
refers
to
how
close
together
a
group
of
measurements
actually
are
to
each
other.
Precision
has
nothing
to
do
with
the
true
or
accepted
value
of
a
measurement,
so
it
is
quite
possible
to
be
very
precise
and
totally
inaccurate.
In
many
cases,
when
precision
is
high
and
accuracy
is
low,
the
fault
can
lie
with
the
instrument.
If
a
balance
or
a
thermometer
is
not
working
correctly,
they
might
consistently
give
inaccurate
answers,
resulting
in
high
precision
and
low
accuracy.
A
dartboard
analogy
is
often
used
to
help
students
understand
the
difference
between
accuracy
and
precision.
Imagine
a
person
throwing
darts,
trying
to
hit
the
bull's--eye.
The
closer
the
dart
hits
to
the
bull's--eye,
the
more
accurate
his
or
her
tosses
are.
If
the
person
misses
the
dartboard
with
every
throw,
but
all
of
their
shots
land
close
together,
they
can
still
be
very
precise.
You
must
strive
for
both
accuracy
and
precision
in
all
of
your
laboratory
activities
this
year.
Make
sure
that
you
understand
the
workings
of
each
instrument,
take
each
measurement
carefully,
and
recheck
to
make
sure
that
you
have
precision.
Without
accurate
and
precise
measurement
your
calculations,
even
if
done
correctly,
are
quite
useless.
THE
SHORT
ANSWER:
ACCURACY
--how
close
you
are
to
an
accepted
value
--IN
GRAPHS:
how
close
your
average
(or
sometimes
slope)
is
to
the
true
value
PRECISION
--how
close
your
measurements
are
to
each
other
--refers
to
the
uncertainty
in
the
measurement
IN
GRAPHS:
how
close
all
your
data
points
are
to
the
best
fit
line
SEE
p.
75
in
Heath:
Figure
at
the
bottom
CHEMISTRY
11
UNCERTAINTY
There
is
no
such
thing
as
a
______________________________.
Each
measurement
contains
a
degree
of
____________________
due
to
the
limits
of
_______________________________.
In
laboratory
exercises,
students
are
expected
to
follow
the
same
procedure
that
scientists
follow
when
they
make
measurements.
Each
measurement
should
be
reported
with
some
digits
that
are
_______________
plus
one
digit
with
a
value
that
has
been
________________________.
UNCERTAINTY
IN
MEASUREMENTS
eg.
ruler
graduated
cylinder
beaker
scientific
balance
electronic
balance
The
uncertainty
is
usually
______________________
on
the
glassware
or
equipment
for
eg.
a
grad
cylinder
marked
every
1
ml
would
have
an
uncertainty
of
0.5
ml
On
a
digital
instrument,
the
______________________________________________
is
usually
listed
on
the
instrument.
TWO
MAIN
CAUSES
OF
UNCERTAINTY
IN
THE
LAB:
1.
________________________________________________________________
2.
________________________________________________________________
HOW
UNCERTAINTY
RELATES
TO
YOUR
CHEMISTRY
LABWORK:
1.
________________________________________________________________
2.
________________________________________________________________
3.
________________________________________________________________
Two
concepts
that
have
to
do
with
measurements
are
accuracy
and
precision:
ACCURACY
The
accuracy
of
the
measurement
refers
to
________________________________
____________________________________________________________________
For
example,
if
you
used
a
balance
to
find
the
mass
of
a
known
standard
100.00
g
mass,
and
you
got
a
reading
of
78.55
g,
your
measurement
would
_______________
__________________________________.
One
important
distinction
between
accuracy
and
precision
is
that
_______________
____________________________,
while
precision
can
only
be
determined
________
__________________________________.
PRECISION
Precision
refers
to
_____________________________________________________
________________.
Precision
has
nothing
to
do
with
the
__________________
of
a
measurement,
so
it
is
quite
possible
to
be
__________________________________.
In
many
cases,
when
precision
is
high
and
accuracy
is
low,
the
fault
can
lie
with
the
instrument.
If
a
balance
or
a
thermometer
is
not
working
correctly,
they
might
consistently
give
inaccurate
answers,
resulting
in
high
_________
and
low
________.
A
dartboard
analogy
is
often
used
to
help
students
understand
the
difference
between
accuracy
and
precision.
Imagine
a
person
throwing
darts,
trying
to
hit
the
bull's--eye.
The
closer
the
dart
hits
to
the
bull's--eye,
the
more
______________
his
or
her
tosses
are.
If
the
person
misses
the
dartboard
with
every
throw,
but
all
of
their
shots
land
close
together,
they
can
still
be
very
_________________.
You
must
strive
for
both
accuracy
and
precision
in
all
of
your
laboratory
activities
this
year.
Make
sure
that
you
understand
the
workings
of
each
instrument,
take
each
measurement
carefully,
and
recheck
to
make
sure
that
you
have
___________.
Without
accurate
and
precise
measurement
your
calculations,
even
if
done
correctly,
are
quite
useless.
THE
SHORT
ANSWER:
ACCURACY
--how
close
you
are
to
an
accepted
value
--IN
GRAPHS:
how
close
your
average
(or
sometimes
slope)
is
to
the
true
value
PRECISION
--how
close
your
measurements
are
to
each
other
--refers
to
the
uncertainty
in
the
measurement
IN
GRAPHS:
how
close
all
your
data
points
are
to
the
best
fit
line
SEE
p.
75
in
Heath:
Figure
at
the
bottom
................
................
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