Following this activity, students should be able to:
- Choose the appropriate piece of glassware to ensure the desired level of precision of a particular solution.
- Compare the tolerance of various pieces of glassware.
Sorting out Glassware
- Label the types of glassware below.
- Which pieces of glassware should be labeled “TC” (to contain) or “TD” (to deliver)?
- Although beakers also have markings for volumes, these are not accurate. What characteristic of the glassware pictured above allows for greater accuracy?
- If you wanted to accurately dilute 10.00 mL of one solution to a total volume of 250.0 mL, what steps would you take? Work with your group to come up with a procedure. Be specific about the glassware you would use and how to use it.
- The picture below is a close-up of a buret. What would you record in your lab notebook based on the water level shown?
- Read the below “best practices” for preparing solutions quantitatively. These are considerations that should included when preparing both standards and samples in quantitative chemical analysis.
Making Solutions: Best Practices
- Graduated cylinders are considerably less accurate and precise than glass transfer pipets.
- Measuring (Mohr) pipets are less precise than glass transfer pipettes.
- Dilution in one step is better than two.
- Larger glassware has less relative uncertainty.
- Glassware is designed to hold a specific volume only at the temperature printed on it (usually 25°C).
- Discuss the list above with your group. Suggest at least three things that could be added.
- What if a student didn’t know that volumetric glassware is most accurate at room temperature? If he or she diluted a cold stock solution to prepare a standard, would the concentration of the resulting diluted solution be erroneously high or low? Explain.
- Suppose a student needs a 15.00-mL pipet to make one of her standards, but only has 5.00-mL, 10.00-mL and 20.00-mL Class A transfer pipets available in her lab. Which action do you recommend for preparing the solution?
- Omit the solution from her set of calibration standards.
- Use a 20.00-mL pipet to make solution E, updating her lab notebook to reflect the change in concentration.
- Use a 20.00-mL pipet to make solution E, but use a 5.00-mL pipet to remove the excess before diluting to volume in the flask.
- Use a 10.00-mL pipet and a 5.00-mL pipet together.
Tolerance is the permissible deviation from a specified value. All volumetric glassware has some tolerance for accuracy – that is, all glassware contains or delivers volumes that can be slightly different from the stated volume that is printed on the glassware. This (incomplete) table shows the tolerances for Class A volumetric flasks.
% Relative Tolerance
- Class A glassware is more accurate that Class B. Would you expect the tolerance of a Class B flask to be greater or less than that of a Class A?
- Complete the missing values for % relative tolerance. What happens to the % relative tolerances as the volume of the flask increases?
- The lower relative uncertainty for larger glassware may seem to suggest it is always better to prepare large volumes of solutions. Can you think of any reasons you would not want to carry out an analysis that required the preparation large volumes of solutions?