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 the balances and thermometers
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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.
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 reportBackground
- This is the curve formed by the liquid: you must be eye-level with the meniscus to read it properly
5 mL: 5.00 mL 10 mL: 10.00 mL +/- 0.01 mL 25 mL: 25.00 mL +/- 0.01 mL 100 mL: 100.0 mL +/- 0.1 mL
1) Relating volume and density
Suppose you have measured the mass of H2O and have looked up its density in the CRC Handbook at a measured temperature
density = mass / volume
Example: what is the volume if the mass of H2O is 25.032 g and its density is 0.99976 g/mL?
25.032 g x 1 mL = 25.038 mL
2) Percent error
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.
% error = (measured - expected) x 100%
Example: you have H2O, contained in a 25-mL volumetric flask, whose volume you have calculated to be 24.752 mL. What is the percent error?
% error = (measured – expected) x 100%
A 25-mL volumetric flask contains 25.00 mL, the “expected” value.
% error = (24.752 – 25.00) mL x 100%
Note the critical
sig figs in red
% error = -0.25 mL x 100% = -0.99%
Pay attention to sig figs in this lab!!