 Systematic (determinate) error – error that can be discovered and correct
ex: pH buffer
is labeled 7.00 pH when in actuality it is 7.08 pH.
 Random (indeterminate) error – arises from limitations in ability to make physical
measurements
ex: you are reading a scale and the mass is between 1.4g and 1.5g and u
have to estimate the last significant figure
 Significant figures – refers to the digits known with certainty plus the first estimated
digit
 Precision – the repetition of a measurement several times or the reproducibility of a
measurement
 Accuracy – nearness to the true measurement
 Normal (Gaussian) distribution – characterized with the most probable answer in the
center and the probability of other answers decreases and the distance from the center
increases. This type of distribution when concerning a lot of different values is often
plotted on a bellshaped curve.
 Mean (xbar, average) – the sum of the measured values divided by the number of
measurements.
 Standard deviation – a measure of the width of the distribution. The scatter of measured
values about a mean (or deviations from the mean) is a measure of precision. The smaller
the standard deviation the more precise the measurement.
(
29
1
2


=
∑
n
x
x
s
i
i
 Degrees of freedom – (n – 1) in the standard deviation equation. After computing the
mean there are only (n – 1) independent pieces o information left.
 In a normal distribution mean ± 1s 68.3% of observations; mean ± 2s 95.5%
observations; mean ± 3s 99.7% observations
 A result more than ± 2s from the mean should be discarded.
 Q test – a statistical method used to test if a point should be rejected. To apply method
arrange data in increasing order and use this formula
range
gap
Q
=
 The gap is the distance from the questionable point and the nearest value and the range
is the range of all the values. Q is then compared to a chart which based on the number of
observations tells you if Q should or should not be discarded.
 Tare – the weight of a container deducted from the total weight of the container and its
contents.
 Mass by difference – this method determines the weight of a sample by subtracting the
amount of sample removed from the weighing container rather than adding it.
 Analytical balances is sensitive within ± 0.003g
 Water has surface tension which means that water molecules have a greater attraction to
the molecules in the glass than to other water molecules.
 Meniscus – the curved surface of water in glassware caused by the wetting of the glass
walls by the solution. Water volumes are to be read at the bottom of the meniscus.
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 Because of the surface tension of water some glassware is calibrated to contacting (TC)
and other glassware is calibrated to deliver (TD) a certain volume of water. This is turn is
also calibrated for a certain temperature because the density of water changes as
temperature changes.
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