Determining Mass
Mass is a direct measurement of
the amount of matter in a sample.
1
Multi Beam Balance
2
Here are the steps for using a multi-beam balance
properly:
•
•
•
Adjust all riders, so that
each reads zero.
Then check to make
sure that the pointer, or
beam, swings to "zero"
on the center mark.
If the pointer does not
read correctly, then use
the "adjustment screw"
to obtain the zero
reading.
3
Using a Multi-Beam Balance
• Warning: do not place any chemicals onto the
balance pan or platform.
– All chemicals, solutions, or compounds should be
placed into a beaker or other container first
• After you place the sample onto the pan or
platform, move the largest rider across the beam
until the pointer drops.
– Then move this rider back once.
• Follow this same procedure for the rest of the smaller riders,
until the pointer swings to the zero mark.
• To record the mass, add the masses shown by
each rider. (Note: masses are in grams)
4
Answer: 165.32g
5
Practice Time
6
Double Pan Balance – Need known
masses to determine the mass
7
Kind You Will Use Later
8
Electronic Balance
Accurate to .01 grams
Usually used to weigh
solids
Have 2 different kinds
9
Analytical Balance
Accurate to 0.001 grams
10
Using a Digital Balance
1. Sometimes when nothing is on the
balance pan, the mass display will show
a non-zero reading.
2. Taring is used to set the mass display to
zero.
3. Once the balance has been tared, the
mass of any object set on the balance
pan will be displayed on the digital
readout.
11
Accuracy versus Precision
How “Good” is a Mass
Measurement
12
Accuracy
= of a measurement is how close that
measurement is to the true or “exact”
value
EX: Standard weight = 5.00g
4.98g more accurate than 5.12 g
13
Precision
• Precision = making reproducible or
repetitive measurements of the same
quantity
• How fine the divisions are
• There will always be some uncertainty
because of the limits in the accuracy of
your instruments
14
Absolute Value
= considered the amount of mass difference
between zero and the determined mass
- are determining how for “off” the standard
the mass is
- recorded as a positive answer even if the
value resulted in a negative number
15
Value of Error in Minting Pennies
•How close the individual volumes are to each
other
•Calculate as follows
% error = calculated volume - actual volume
actual volume
•Good accuracy is indicated by a relatively low
error value.
16
Example:
% error in minting pennies = average volume - actual volume
actual volume
% error in minting pennies = 1.2g - 1.4g
1.4g
= 0.14 X 100%
= 14 %
So there is a 14% error in minting of these particular pennies
17
Determining the % Accuracy of a
Balance
• Going to calculate the % accuracy of the
balance you are using
% Accuracy = experimental value X 100%
reference or standard value
The closer the experimental value is to the
reverence or standard value, the greater
the accuracy.
18