2.2: Exercise
A. Volume
The Effect of Glass and Plastic on the Meniscus
1) Obtain a 10 mL pipette and a beaker of colored liquid. Transfer 8 mL of the colored liquid into a 100 mL glass graduated cylinder. Repeat 4 times.
a. What volume of liquid should the cylinder contain? __________
b. What volume of liquid does your graduated cylinder contain? (Remember that the bottom of the meniscus marks the correct volume) __________
c. What could contribute to a difference?
2) Repeat step 1 but this time transfer the liquid into a 100 mL plastic graduated cylinder instead. Are there any differences? Yes / No Explain :
The Micropipette
1) Use a micropipette (p200) to transfer 25.5 µL of red dye solution onto a small piece of Parafilm or wax paper. After dispensing the dye, use the same pipette to pick up the drop and transfer it to a different location. Repeat the step with a new micropipette (p20) and 13.7 µL of red dye solution.
2) Are you able to pick up the entire drop of liquid each time? Yes / No Explain:
The Graduated Cylinder
1) Partially fill a 100 mL glass graduated cylinder with tap water and set it on your table. View the cylinder and measure the volume of the water while
(a) looking down at the cylinder __________, (b) at eye level _________, and (c) below and looking up at the cylinder ___________.
Is there any difference between the three? Yes / No Explain: _________________________________________________
2) Transfer the water to a plastic graduated cylinder. View the cylinder and measure the volume of the water while (a) looking down at the cylinder __________,
(b) at eye level ____________, and (c) below and looking up at the cylinder _____________.
Can the volume be read more precisely in the glass or plastic cylinder? Glass / Plastic
3) Add tap water to the 10 mL level in a 100 mL glass graduated cylinder. Then, transfer this water into a 10 mL graduated cylinder.
What measurement do you read on the smaller cylinder? ___________ Which cylinder would be more accurate for measuring 5 mL of solution? ______________________
The Volumetric Flask
Your instructor will demonstrate this activity .
· Instructor: put a few drops of dye into the volumetric flask and add water until it reaches the 100 mL mark on the volumetric flask, use a dropper to be as accurate as possible.
· Pour the water into a graduated cylinder. Is it more, less than, or equal to, the 100 mL mark? _____________
· Instructor: Now, pour this water into an Erlenmeyer flask and estimate the volume: ______________mL
B. Precision versus Accuracy
- Which device is more accurate? Volumetric flask / Graduated cylinder / Erlenmeyer flask
- Which device is more precise? Volumetric flask / Graduated cylinder / Erlenmeyer flask
Preparing Solutions
- Circle the concentration assigned to your group: 0.4, 0.6, 0.8, 1.0, 1.2, 1.4. Show your entire calculation below, calculate how many grams of NaCl is required to make your assigned molarity. Then, prepare 100 mL of NaCl solution of your assigned molarity.
2. Place a drop of water on the hand refractometer to measure the specific gravity of water. Record the specific gravity of the water drop: ___________, then dry the refractometer with a KimWipe. Next, place a drop of your assigned NaCl solution onto the hand refractometer and record the specific gravity: _____________. Clean with water and dry with a KimWipe.
3. Go around the room and measure the specific gravity for each group's solution. Clean with water and dry with a KimWipe after each use. Record the data in the table below, include your group’s solution too.
|
Molarity of NaCl solution |
Specific gravity |
|---|---|
|
0.4 M |
|
|
0.6 M |
|
|
0.8 M |
|
|
1.0 M |
|
|
1.2 M |
|
|
1.4 M |
Preparing Serial Dilutions
Serial dilutions- a stepwise dilution of a substance in solution. We use serial dilutions to produce low molarity solutions more accurately. Use your original assigned molarity as the stock solution.
- Measure 9 mL of water and transfer into a beaker or test tube. Then, transfer 1 mL of your stock solution into the 9 mL of water and mix well. This is your first dilution. (1/10 dilution)
- Next, measure 9 mL of water and transfer into a new beaker or test tube, then transfer 1 mL of your first dilution into this new 9 mL of water and mix. This is your second dilution. (1/10 * 1/10 = 1/100 dilution)
- Use the hand refractometer to measure the specific gravity of each of the two dilutions and record in the table below.
- Does s pecific gravity increase or decrease as you dilute a solution? ________________
|
NaCl stock solution |
Molarity ________M |
Specific gravity: |
|---|---|---|
|
First dilution |
Molarity ________M |
Specific gravity: |
|
Second dilution |
Molarity ________M |
Specific gravity: |