Lab: Accuracy and Precision
Introduction: The accuracy of your measurements and the precision of your measuring instruments both play
an important part in your ability to get the best results during a laboratory experiment. In this lab, we will
explore the precision of several pieces of lab equipment, and you will determine your own ability to be accurate
when measuring.
Materials:
Electronic balance
Buret setup
50 mL Beaker
Tap water
50 mL Graduated cylinder
Distilled water
Procedure 1:
At the stock table you will find a display of several graduated cylinders filled with different amounts of colored
water and other solutions. Read each graduated cylinder and fill in your answers in data table below.
Graduated
cylinder
Color of the liquid
To what unit does the smallest
scale mark read?
(1’s place, .1’s place, etc)
Your reading
(mL)
1
2
3
Procedure 2:
1. At your lab station, clean and wipe dry your 50 mL graduated cylinder and your 50 mL beaker.
2. Weigh (separately) the graduated cylinder and the beaker. Record the masses.
3. Using the graduated cylinder, fill the 50 mL graduated cylinder so that the meniscus lines up with the 25
mL mark. Make sure you have EXACTLY 25 mL of water in your cylinder.
4. Using the beaker, fill the beaker up to the 25 mL mark. Make sure you have EXACTLY 25 mL in your
beaker.
5. Weigh (separately) both the filled graduated cylinder and the filled beaker. Record the masses.
6. Determine the mass of water in the graduated cylinder and the beaker.
7. Repeat steps 1-6 two more times.
8. Fill out the Data Table below and be sure to include units for EVERY measurement.
Graduated Cylinder (include units)
Trials
1
2
3
Mass of empty
cylinder (g)
Mass of filled
cylinder (g)
Mass of water
(g)
Volume of water
(mL)
1. What is the average mass of the 25 mL’s of water in your cylinder? ______________
(average the Mass of water from the data table above)
Because the density of water is 1 gram for every 1 mL, a 1-1 mass-volume equivalence can be found
from the weighed water. For example, 50.0 g of water would be equivalent to 50.0 mL of water.
(Caution: this one-to-one ratio only works for WATER.)
2. Using your answer from question 1, what is the actual volume of the 25 mL’s of water in your cylinder?
__________________
Beaker (include units)
Trials
Mass of empty
beaker (g)
Mass of filled
beaker (g)
Mass of water
(g)
Volume of water
(mL)
1
2
3
3. What is the average mass of the 25 mL’s of water in your beaker? ______________
(average the Mass of water from the data table above)
Because the density of water is 1 gram for every 1 mL, a 1-1 mass-volume equivalence can be found
from the weighed water. For example, 50.0 g of water would be equivalent to 50.0 mL of water.
(Caution: this one-to-one ratio only works for WATER.)
4. Using the answer from question 3, what is the actual volume of the 25 mL’s of water in your beaker?
__________________
Procedure 3:
1.
2.
3.
4.
5.
6.
7.
8.
Fill the buret to a point somewhat below the zero mark (between 0.00 and 1.00) with distilled water.
Record the initial volume.
Clean and wipe dry a 50 mL beaker. Record the mass of the beaker.
Place the beaker under the stopcock of the buret and dispense the water into the beaker until
approximately 25 mL of water has been dispensed. YOU MUST BE WATCHING the buret level drop to
measure the 25 mL. DO NOT judge the 25 mL by looking at the beaker!
Record the final volume.
Reweigh the beaker and determine the mass of the water transferred.
Repeat this procedure one more time.
Fill out the data table below and be sure to include units for EVERY measurement.
Buret (include units)
Trial
Initial buret
reading
(mL)
Final buret
reading
(mL)
Buret
Volume (mL)
(F – I)
(expt.
Volume)
Mass of
beaker
(g)
Mass of
beaker and
water (g)
Mass of
water (g)
(actual
volume)
1
2
Actual vs. Experimental data
When you filled your graduated cylinder and your beaker to the 25 mL mark, you observed this to be “exactly”
25 mL. This is your experimental data. Using a balance to calculate the mass and volume of the water gives
you the actual or most accurate and accepted value for the mass and volume of the water because you used a
tool much more precise than your eyes. Using this data, you can calculate the percent error of the beaker and
of the graduated cylinder.
Percent error Calculations
Formula:
Accepted (actual) value – experimental value
x 100
Accepted value
**Notice that the numerator is inside absolute value bars.
Calculations:
1. Based on the information above and using your answer from question #2 as the accepted value and 25 mL
as the experimental value, calculate the percent error for your graduated cylinder. (show your work for
credit)
2. Based on the information above and using your answer from question #4 as the accepted value and 25 mL
as the experimental value, calculate the percent error for your beaker. (show your work for credit)
3. Calculate the percent error for the each buret trial using the buret volume as your experimental value and
the mass of the water as your accepted value. (show your work for credit)
Trial 1:
Trial 2:
Analysis Questions:
1. Put your percent error for the beaker on the board with the rest of the class data. Find an average
percent error for the beaker. _______________
2. Put your percent error for the graduated cylinder on the board with the rest of the class data. Find an
average percent error for the graduated cylinder. ________________
3. Put your percent error for the buret on the board with the rest of the class data. Find an average percent
error for the buret. _________________
4. How does the way a piece of glassware is divided into scale marks (the glassware’s precision) affect the
accuracy of your answers? Compare the beaker’s markings, the graduated cylinder’s markings, and the
buret’s markings.
5. Compare the RANGE of your answers for each piece of glassware in the table below (find the greatest
difference between your experimental and actual numbers).
Beaker range
(Accepted – Expt.)
Graduated cylinder range
(Accepted – Expt.)
Buret range
(Accepted – Expt.)
6. The range of your answers tells us about the precision of your measuring instrument (glassware, in this
case!) A small range means good precision. Which piece of equipment gave you the most precise
answers? JUSTIFY your answer with an explanation.
7. When measuring liquids in the lab, explain which piece of equipment will help you to get the most
accurate and precise answers (and therefore make your lab come out better!): the graduated cylinder, or
the beaker?