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Final Review - Systematic(determinate error error that can...

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- 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 1.4-g and 1.5-g 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 bell-shaped curve. - Mean (x-bar, 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 ± 0.003-g - 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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