Volumetric Measurements In addition to this presentation, before coming to lab or attempting the prelab quiz you must also:  Read chapter IV in the “Laboratory Handbook…” by Griswold et al. before coming to lab  Watch the prelab videos on the use of thebalances and thermometers  Read the introduction to the lab exercise in the coursepack Note: This presentation specifies the precision of all measuring devices you will use in this lab (i.e., how many sig figs each instrument allows). This is useful information to write in your lab notebook, near the front.

What’s the point? In this lab you willIn this lab you will gain experience using volumetric glassware - graduated cylinders, beakers, volumetric flasksgain experience using volumetric glassware - graduated cylinders, beakers, volumetric flasks gain a sense of the precision and accuracy of different glasswaregain a sense of the precision and accuracy of different glassware learn to relate significant figures in reported measurements to precision in measuring deviceslearn to relate significant figures in reported measurements to precision in measuring devices practice working with significant figures and units - see Chapter 1, Brown, LeMay and Bursteinpractice working with significant figures and units - see Chapter 1, Brown, LeMay and Burstein

Background Different pieces of volumetric glassware have different purposes and therefore different precision and accuracyDifferent pieces of volumetric glassware have different purposes and therefore different precision and accuracy this is true of all measuring glassware; compare a measuring cup and a cake mixing bowl with volume marks The difference in precision and accuracy affect three important things: - how reproducible the measurements are - how accurate the measurements are - how many significant figures you can reportThe difference in precision and accuracy affect three important things: - how reproducible the measurements are - how accurate the measurements are - how many significant figures you can report

Graduated Cylinders Use: to measure an amount of liquid to transferUse: to measure an amount of liquid to transfer Depending on the cylinder size, they have markings for every 1 mL, 2 mL, 0.1 mLDepending on the cylinder size, they have markings for every 1 mL, 2 mL, 0.1 mL Read the volume by finding the bottom of the meniscusRead the volume by finding the bottom of the meniscus - This is the curve formed by the liquid: you must be eye-level with themeniscus to read it properly In the figure, the markings are every 2 mL, but you can read between the marks to get +/- 1 mL precision (i.e., the volume here would be reported as 100 mL, not mL)In the figure, the markings are every 2 mL, but you can read between the marks to get +/- 1 mL precision (i.e., the volume here would be reported as 100 mL, not mL)

To use a graduated cylinder:To use a graduated cylinder: fill the cylinder to the desired markfill the cylinder to the desired mark position yourself eye-level with the meniscusposition yourself eye-level with the meniscus read the volume from the bottom of the meniscusread the volume from the bottom of the meniscus now, you can be confident that you are transfering the desired amount of liquidnow, you can be confident that you are transfering the desired amount of liquid

Beakers Use: to transfer approximate amounts of liquidsUse: to transfer approximate amounts of liquids Beakers have markings that are widely spaced. So, the volume readings are approximateBeakers have markings that are widely spaced. So, the volume readings are approximate In the figure, you see two volume scales. On the right is the amount in the beaker.In the figure, you see two volume scales. On the right is the amount in the beaker. Also in the figure, you can see this beaker’s precision is given as +/- 5%.Also in the figure, you can see this beaker’s precision is given as +/- 5%. For a 250 mL beaker, that is:(0.05 x 250 mL) = 12.5 mL = +/- 10 mLFor a 250 mL beaker, that is:(0.05 x 250 mL) = 12.5 mL = +/- 10 mL So, here we report the volume to the tens place: 200 mL, or 190 mL, but not 195 mLSo, here we report the volume to the tens place: 200 mL, or 190 mL, but not 195 mL

Volumetric Flasks Use: to contain an accurate amount of liquidUse: to contain an accurate amount of liquid Volumetric flasks have one mark that indicates the calibrated volume at a particular temperature (e.g., 20 o C)Volumetric flasks have one mark that indicates the calibrated volume at a particular temperature (e.g., 20 o C) Virtually all are made to contain (see introduction to the lab exercise)Virtually all are made to contain (see introduction to the lab exercise) When the bottom of the meniscus is level with the mark, volumetric flasks have the following precisions:When the bottom of the meniscus is level with the mark, volumetric flasks have the following precisions: 5 mL: 5.00 mL 10 mL: mL +/ mL 25 mL: mL +/ mL 100 mL: mL +/- 0.1 mL

Volumetric flasks are very useful for making solutions of known volume.Volumetric flasks are very useful for making solutions of known volume. In this lab, we will be using them simply to investigate their accuracy and precision.In this lab, we will be using them simply to investigate their accuracy and precision. To use a volumetric flask: - fill the cylinder to the desired mark - position yourself eye-level with the meniscus - read the volume at the bottom of the meniscus - now, you can be confident that the flask contains the desired amount of liquidTo use a volumetric flask: - fill the cylinder to the desired mark - position yourself eye-level with the meniscus - read the volume at the bottom of the meniscus - now, you can be confident that the flask contains the desired amount of liquid

Necessary Calculations Suppose you have measured the mass of H 2 O and have looked up its density in the CRC Handbook at a measured temperature density = mass / volume 1) Relating volume and density

mass = g density = g/mL g x 1 mL = mL g Example: what is the volume if the mass of H 2 O is g and its density is g/mL?

Necessary Calculations Suppose you have measured something for which you know the true or expected value. How accurate is your measurement? Is it too small (negative error) or too big (positive error)? Percent error is one way to express this accuracy. 2) Percent error % error = (measured - expected) x 100% expected

% error = (measured – expected) x 100% expected A 25-mL volumetric flask contains mL, the “expected” value. % error = ( – 25.00) mL x 100% mL % error = mL x 100% = -0.99% mL Example: you have H 2 O, contained in a 25-mL volumetric flask, whose volume you have calculated to be mL. What is the percent error? Note the critical sig figs in red Pay attention to sig figs in this lab!!

For Everyone’s Safety your lab goggles and coat must be on whenever anyone is working in the labyour lab goggles and coat must be on whenever anyone is working in the lab no open toed shoes are allowed in labno open toed shoes are allowed in lab move around the lab carefully to avoid bumping into other studentsmove around the lab carefully to avoid bumping into other students