Accuracy and Precision of

Various Density Measurements

Introduction:

Mass and volume are extensive properties which depend on the quantity of substance. Density, which is the ratio of mass to volume (D=m/V), is an intensive property. The density of a substance, regardless of sample size, is a constant and is characteristic of the substance. Therefore, density can be used to aid in the identification of the substance. However, the density of a substance may vary with its temperature. In this lab the mass and volume of water will be measured, and the density of the water will be calculated. This will be compared to the accepted density of water at the temperature reported. Both accuracy and precision of the measurements will be determined, keeping in mind that an accurate measurement is one that is close to the correct value and precise measurements agree with one another.

Purpose:

To become familiar with different types of balances and volume-measuring devices by measuring the volume and mass of water samples, determining the density of water and calculating the accuracy of the density determination and the precision of the density determination.

Materials:

thermometer / micropipet / 50 mL Beaker
250 mL Beaker / Top loading balance / 10 mL Graduated cylinder
distilled water / Analytical balance / volumetric Pipet

Note: Equipment substitutions may be made. For example, a triple beam balance may be substituted for a toploading balance or a graduated plastic pipet may be substituted for the 1mL volumetric pipet. Tap water may be used if distilled water is unavailable.

Safety Considerations:

·  This experiment poses no safety hazards.
It is good laboratory procedure to wear goggles and aprons at all times.

Procedure:

Place about 100 mL of distilled water in a 250 mL beaker, called the supply beaker. Allow the water to come to room temperature. Measure the temperature of the water and record it on the data table. Use this water for all of the measurements below. Follow this procedure for BOTH the toploading and the analytical balance. Record all measurements to the number of decimal places allowed by the measuring device.

Beaker:

1.  Place a weigh boat on the balance. Tare.

2.  Pour 1 ml of water from the supply beaker in to the 50 ml beaker, using only the markings available. Put the water from the 50 ml beaker into the tared weigh boat. Record the mass of water added to the weigh boat. Tare.

3.  Measure another ml of water into the 50 ml beaker. Add this ml of water to the weigh boat in the same manner as above. Record the mass of water added to the weigh boat. Tare.

4.  Repeat step two one more time. Record the mass of water added to the weigh boat.

Graduated cylinder:

1.  Place a weigh boat on the balance. Tare.

  1. Using the 10 mL graduated cylinder as the measuring instrument for volume, add 1 mL of water from the supply beaker to the tared weigh boat. Record the mass of water added to the weigh boat. Tare.

3.  Measure another 1 mL of water using the graduated cylinder and add to the weigh boat. Record the mass of water added to the weigh boat. Tare.

4.  Repeat step two one more time. Record the mass of water added to the weigh boat.

Pipet:

1.  Place a weigh boat on the balance. Tare.

2.  Using the volumetric pipet as the measuring instrument for volume, add 1 mL of water from the supply beaker to the tared weigh boat. Record the mass of water added to the weigh boat. Tare.

3.  Add another 1 mL of water to the weigh boat. Record the mass of water added to the weigh boat. Tare.

4.  Add another 1 mL of water to the weigh boat. Record the mass of water added to the weigh boat.

Micropipet:

1.  Place a weigh boat on the balance. Tare.

2.  Using the micropipet as the measuring instrument for volume, add 1 mL of water from the supply beaker to the weigh boat. Record the mass of water added to the weigh boat. Tare.

3.  Add another 1 mL of water to the weigh boat. Record the mass of water added to the weigh boat. Tare.

4.  Add another 1 mL of water to the weigh boat. Record the mass of water added to the weigh boat.


Name______

Data:

Temperature of water ______oC Accepted density of water at this temp. ______

Equipment used / Volume (ml) / H2O Mass (g) / Density (g/ml) / Average Density (g/ml)
Beaker on toploading balance
Graduated Cylinder on toploading balance
Pipet on toploading balance
Micropipet on toploading balance
Beaker on analytical balance
Graduated Cylinder on analytical balance
Pipet on analytical balance
Micropipet on analytical balance

Calculations/Results:

1. Calculate the % error (accuracy) for each average density value.

% error = |accepted value – your calculated average| x 100

accepted value

% ERROR for: Toploading Analytical

Beaker
Graduated Cylinder
Pipet
Micropipet

Name______

Name______

Period______Date______

Accepted Values for the Density of Water

Modern Chemistry , copyright 1990, page 829

Temperature / Density / Temperature / Density
°C / g/mL / °C / g/mL
0 / 0.99984 / 24 / 0.99730
2 / 0.99994 / 26 / 0.99678
3.98 (max) / 0.999973 / 30 / 0.99565
4 / 0.99997 / 40 / 0.99222
6 / 0.99994 / 50 / 0.98804
8 / 0.99985 / 60 / 0.98320
10 / 0.99970 / 70 / 0.97777
14 / 0.99924 / 80 / 0.97179
16 / 0.99894 / 90 / 0.96531
20 / 0.99820 / 100 / 0.95836

Questions:

1.  Which method of determining density was most accurate? How do you know?

2.  Which method of determining density was most precise? How do you know? Could you have made the same determination by inspection (looking at the data)?

3.  Did the mass shown on the analytical balance tend to decrease with time? If so, why? If not, why not?

4.  Density can be determined mathematically and graphically. What are the advantages of each method? What are the disadvantages of each method?

Extension:

1. Calculate the precision (standard deviation) of each set of data.

standard deviation =

where X = density for each individual trial

Xave= average density

n = number of trials

Precision for: Toploading Analytical

Beaker
Graduated Cylinder
Pipet
Micropipet

ASIM Accuracy and Precision 5

Revised: 10/09