LAB 1 Flashcards
What two quantities can we use t characterize the reliability of experimental measurements?
accuracy and precision
what is accuracy
Accuracy measures the extent to which measurements agree with a known value.
HOW CLOSE IT IS TO THE VALUE
what is precision
Precision measures the consistency of the measurements with each other, independent of their accuracy.
Error can be grouped into two types:
random and systematic
Random error (indeterminate) is observed when
repeated experiments give slightly different results. result of the limitations of instruments to produce the same result each time.
Data that has a minimal amount of random error
is said to be precise.
Systematic error
(determinate or bias) is observed when an experiment produces results that are consistently too high or too low.
- result of instruments not properly calibrated or an experimental design that is flawed
Data that has minimal amount of systematic error
is said to be accurate.
Precision is often expressed statistically by the
standard deviation (𝑠) of a series. A standard deviation close to zero indicates that the data points are very close to the mean.
KNOW FORMULA
what does xi represent
individual measured values
x̅ is
is the mean
n is
number of measured values in the series.
x̅±
Results are expressed as x̅ ± standard deviation
If a ‘true’ or ‘accepted’ value is known
then accuracy can be expressed as the relative percent error:
Volumetric glassware are used when
(glassware used to measure liquid volumes), such as volumetric flasks, pipets, and burets, are used when measurements are required to a high degree of accuracy and precision.
For example, a typical 10-mL pipet used in the lab has a tolerance of
± 0.02 and the volume is recorded as 10.00 mL.
When using graduated cylinders,
you need to determine the value of the smallest increment and then always estimate the measurement one decimal place beyond that value, even if the last estimated digit is a zero.
Beakers and flasks are not usually used as
measuring devices and their markings are generally only accurate to within 5%.
volume measured with
(using beakers, graduated cylinders, and pipets)
mass measured using
scales
relationship between volume and mass
. Density, d, is the amount of mass, m, per unit volume, v, of a substance.
Density can be used to
- describe a pure substance (element/compound) or solution
- density measurements are affected by changes in temp and pressure
- for solution, density also changes with solution concentration
calibration curve
how density changes as a function of concentration from which densities can be predicted at other concentrations, or vice versa.
Figure 1
there is a linear relationship between density and concentration. for NaCl
Here the equation of the line is given by d = 0.0076c + 0.9988 (where d = density and c = %w/w concentration). Thus, if we have a salt solution which is 12.0% w/w then the calculated density is (0.0076 × 12.0) + 0.9988 = 1.09 gcm-3.
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