Volume Determination and Density Lab 3

Outline Reading a meniscus Volume determination and significant digits Balances Mathematical treatment of data Safety Concerns Next Assignment Summary of Significant Figure Rules for Glassware

Reading a Meniscus Meniscus and Parallax

Very Important Every time you read a volume from a piece of glassware in lab, or report a volume for a calculation, you need to use the appropriate level of precision of the glassware used. Study the following rules and use them correctly and consistently.

Significant Figures: Volumetric Glassware Use the Tolerance table on p. 98 to determine glassware tolerance, unless it is printed on the glassware. For example: The 50 mL volumetric flask has a tolerance of ± 0.05 mL. Whenever you use your 50 mL volumetric flask, you will always report two zeros after the decimal, i.e mL.

Volumetric Flask When this volumetric flask is filled to the calibration mark, what volume should be reported?

Volumetric Pipet When this pipet is used to dispense solution, what volume should be reported?

Volumetric Pipet When this pipet is used to dispense solution, what volume should be reported? And to which line should the pipet be filled?

Significant Figures: Graduated Glassware 1. Look at the glassware in question. 2. Find the smallest graduation. 3. Report your volume to the closest 10% of the smallest graduation.

Reading Graduated Glassware

Smallest Graduation A. Calculate major graduation – major graduation B. Determine the number of spaces between two major graduations Smallest Graduation = A / B

Reading Graduated Cylinders What volume should be reported?

Reading Graduated Cylinders Smallest graduation: 0.2 mL 10% value: 0.02 mL Volume to the closest 10% of the SG: 6.62 mL

Reading Graduated Cylinders What volume should be reported?

Reading Graduated Cylinders Smallest graduation: 5 mL 10% value: 0.5 mL Volume to the closest 10% of the SG: mL

Reading Graduated Cylinders What volume should be reported?

Reading Graduated Cylinders Smallest graduation: 0.5 mL 10% value: 0.05 mL Volume to the closest 10% of the SG: mL

Reading Burets What volume should be reported?

Reading Burets Smallest graduation: 0.1 mL 10% value: 0.01 mL Volume to the closest 10% of the SG: mL

Reading Burets What volume should be reported?

Reading Burets Smallest graduation: 0.1 mL 10% value: 0.01 mL Volume to the closest 10% of the SG: mL

Balances Cleaning Two methods of weighing Recording of significant figures All sample/solution preparation should be performed at your station. Balances (and related counter space) should be used for weighing only!

Density Mass of a substance per unit volume. Units for today is g/mL. Density changes with changing temperature. As a result, volume changes (but mass stays constant unless you spill).

Mathematical Treatment of Data % Error = x 100%

Example of Volume Calculation Dry mass of a 50 mL beaker: g Exactly mL of water is transferred from a 10 mL graduated cylinder into a 50 mL beaker. Mass of the 50 mL beaker with mL of water: g Mass of 10 mL water: g Volume read: mL (why 2 digits of precision?)

Example of Volume Calculation Water temperature: 21.2 ºC Density at this temperature (p. 100): g/mL Volume Calculation:

% Error Calculation

Safety Concerns Reagent: Food coloring Health Considerations: Avoid contact with skin and eyes. Do not inhale vapor or spray. Do not ingest.

Next Assignment – Lab 4: Read the required reading sections in your textbook and lab manual. Complete and submit your pre-lab questions by the deadline. Remember to study for your quiz. Remember to complete and submit the Lab 3 report at the start of the next lab.

Lab 4 Pre-lab Questions If you haven’t started this pre-lab yet, do so as soon as possible. Read the lab write-up in your manual (pp ). Complete the pre-lab questions (pp ) in MicroLab™ in the SCICom lab (NSB 204), Chemistry Resource Center (CPSB 302K) or on your personal computer if you download the program. The program may be downloaded from under the Downloads link. Take this exercise seriously. You will be required to complete a similar exercise in lab next week without help from your lab manual.

Skill Evaluations – Next Week Will occur after the Lab 4 in-class assignment if time allows. Review the skill evaluation pages for graduated glassware, safety, and calibration curves.

Use these rules for “Volume Read” Graduated Glassware  Look at the glassware  Find the smallest division (graduation)  Report volume to 10% of the smallest division Volumetric Glassware  Find the glassware tolerance on the glassware or in Table 3.1  Report as many zeros after the decimal point for your volume, as digits after the decimal in the tolerance