Measurement in Chemistry

Measurement in chemistry Absolute Error. Absolute Uncertainty. Compare With Relative Error. The uncertainty in a measurement, expressed with appropriate units. For example, if three replicate weights for an object are 1.00 g, 1.05 g, and 0.95 g, the absolute error can be expressed as ± 0.05 g. Absolute error is also used to express inaccuracies; for example, if the "true value" is 1.11 g and the measured value is 1.00 g, the absolute error could be written as 1.00 g - 1.11 g = -0.11 g. Note that when absolute errors are associated with indeterminate errors, they are preceded with "±"; when they are associated with determinate errors, they are preceded by their sign. Absolute Temperature. Temperature measured on a scale that sets absolute zero as zero. In the SI system, the kelvin scale is used to measure absolute temperature. Absolute Zero. (0 K) The temperature at which the volume of an ideal gas becomes zero; a theoretical coldest temperature that can be approached but never reached. Absolute zero is zero on the Kelvin scale, -273.15°C on the Celsius scale, and -459.67°F on the Fahrenheit scale. Accuracy. Compare With Precision And Trueness. Accuracy is the correctness of a single measurement. The accuracy of a measurement is assessed by comparing the measurement with the true or accepted value, based on evidence independent of the measurement. The closeness of an average to a true value is referred to as "trueness". Ampere. (A) Amp. The SI unit of electric current, equal to flow of 1 coulomb of charge per second. An ampere is the amount of current necessary to produce a force of 0.2 micronewtons per meter between two arbitrarily long, arbitrarily thin wires, placed parallel in a vacuum and exactly 1 m apart. Named for 19th century physicist André Marie Ampère. Angstrom. (Å) Ångstrom; Ångstrom Units. A non-SI unit of length used to express wavelengths of light, bond lengths, and molecular sizes. 1 Å = 10-10 m = 10-8 cm. Atto-. (A) Prefix used in the SI system meaning "multiply by 10-18". For example, 3 am means 3× 10-18 meters. Base Unit. Base units are units that are fundamental building blocks in a system of measurement. There are seven base units in the SI system. Calibration. Calibration is correcting a measuring instrument by measuring values whose true values are known. Calibration minimizes systematic error. Celsius. (°C) Celsius Temperature Scale; Celsius Scale. A common but non-SI unit of temperature, defined by assigning temperatures of 0°C and 100°C to the freezing and boiling points of water, respectively. Centi-. (C) Prefix used in the SI system meaning "one hundredth of". For example 1 cm means "one hundredth of a meter"; 2.3 cg could also be written "2.3 × 10-2 g" or "0.023 g". Cgs. Compare With SI. An older metric system of units that uses centimeters, grams, and seconds as base units. Conversion Factor. A conversion factor is a fraction that relates one unit to another. Multiplying a measurement by a conversion factor changes the units of the measurement. For example, since 1 in = 2.54 cm, to convert 10 inches to centimeters, (10 in)= 25.4 cm 2.54 cm 1 in Coulomb. (C) The SI unit of electric charge, equal to the amount of charge delivered by a current of 1 ampere running for 1 second. One mole of electrons has a charge of about 96487 C. Density. (D) Compare With Specific Gravity. Mass of a substance per unit volume. Saying "the density of mercury is 13.55 g/cm3 " is the same as saying "the mass of exactly 1 cm3 of mercury is 13.55 g". Derived Unit. Derived units are units constructed from the SI system's base units. For example, the SI unit for density is kg/m3, derived from the base units kg and m. Dyne. (Dyn) The unit of force in the obsolete cgs system of units. A dyne is the force required to accelerate a 1 g mass by 1 cm/s per second. Femto-. (F) Prefix used in the SI system meaning "multiply by 10-15". For example 22 fg means 22× 10-15 g. Gram. A metric unit of mass, equal to 1/1000 of a kilogram. Kilograms are the base SI units for mass, not grams. Gross Error. Compare With Systematic Error, Random Error And Mistake. Gross errors are undetected mistakes that cause a measurement to be very much farther from the mean measurement than other measurements. Hydrometer. An instrument for measuring the specific gravity of liquids. A hydrometer is a weight with a vertical scale attached. When placed into a liquid, the hydrometer bobs upright, and sinks to a certain level. The specific gravity or solution composition can be read from the liquid level on the vertical scale. Hydrometers are often calibrated in degrees Baumé. Kelvin. (K) The SI base unit of temperature, defined by assigning 273.16 K to the temperature at which steam, ice, and water are at equilibrium (called the triple point of water). The freezing point of water is 273.15 K. Kilo-. (K) Prefix used in the SI system meaning "one thousand of". For example 1 km means "one thousand meters"; 2.8 kg could also be written "2.8 × 103 g" or "2800 g". Limit Of Quantitation. (LOD) Quantitative Detection Limit; Limit Of Determination. The smallest detectable concentration an analytical instrument can determine at a given confidence level. IUPAC defines the quantitative detection limit as Cld = ks/m, where k is 10, s is the standard deviation of instrument readings taken on a "blank" (a solution with zero concentration of analyte), and m is the slope of a plot of instrument response vs. concentration, as calculated by linear regression. Mass. (M) Compare With Weight. Mass is a measure of the tendency of an object to resist acceleration. It's harder to roll a tractor trailer than a roller skate; the tractor trailer has a far greater mass. Measurement. Measurement is the collection of quantitative data. Measurement involves comparison of the quantity of interest with a standard called a unit. The comparison is never perfect. As a result, measurements always include error. You must consider the reliability of the measurement when using it to make decisions or estimate other quantities. Mega-. (M) Mega. SI prefix meaning "multiply by 106". For example, 3.2 MJ is 3200000 J. Meter. (M) Metre. The meter is the basic unit of length in the SI system of units, defined as the distance light travels through a vacuum in exactly 1/299792458 seconds. 1 m = 39.37 inches. Meters are abbreviated as "m" in measurements. Micro-. (Μ) Micro. Prefix used in the SI system meaning "one millionth of". For example 1 µm means "one millionth of a meter"; 3.1 µL means "3.1 × 10-6 L". Milli-. (M) Prefix used in the SI system meaning "one thousandth of". For example 1 mL means "one thousandth of a liter"; 1 mg means "one thousandth of a gram". Mistake. Blunder. Compare With Systematic Error, Random Error And Gross Error. A mistake is a measurement which is known to be incorrect due to carelessness, accidents, or the ineptitude of the experimenter. It's important to distinguish mistakes from errors: mistakes can be avoided. Errors can be minimized but not entirely avoided, because they are part of the process of measurement. Data that is mistaken should be discarded. Data that contains errors can be useful, if the sizes of the errors can be estimated. Nano-. (N) Prefix used in the SI system meaning "multiply by 10-9". For example 1 nm means "0.000000001 m"; 2.8 ng could also be written "2.8 × 10-9 g". Pico-. (P) Prefix used in the SI system meaning "multiply by 10-12". For example, 3 pm means 3× 10-12 meters. Power. The rate at which energy is supplied. Power has define[SI] units of J/s, sometimes called "Watts" (W). Precision. Reproducibility. Compare With Accuracy. Precision is reproducibility. Saying "These measurements are precise" is the same as saying, "The same measurement was repeated several times, and the measurements were all very close to one another". Don't confuse precision with accuracy. Random Error. Indeterminate Error. Compare With Systematic Error, Gross Error And Mistake. Random errors are errors that affect the precision of a set of measurements. Random error scatters measurements above and below the mean, with small random errors being more likely than large ones. Relative Error. Relative Uncertainty. Compare With Absolute Error. The uncertainty in a measurement compared to the size of the measurement. For example, if three replicate weights for an object are 2.00 g, 2.05 g, and 1.95 g, the absolute error can be expressed as ± 0.05 g and the relative error is ± 0.05 g / 2.00 g = 0.025 = 2.5%. Relative Standard Deviation. (RSD) Compare With Standard Deviation. The relative standard deviation is a measure of precision, calculated by dividing the standard deviation for a series of measurements by the average measurement. Second. (S) The second (s) is the base unit of time in the SI system of units, defined as the duration of 9,192,631,770 cycles of the radiation associated with a certain color of light emitted by the cesium atom. Significant Figure. Significant Digit; Significant. A convention for recording measurements. Measurements are rounded so that they contain only the digits up to and including the first uncertain digit, when the number is written in scientific notation. SI. Systeme Internationale; International System. Le Syst�me Internationale (SI) is a system of units introduced to remove barriers to international trade, based on the older metric system. It is now used in science and technical communications worldwide. Specific Gravity. Specific Gravities. Compare With Density. The mass of a unit volume of a substance relative to the mass of a unit volume of water. Temperature must be specified when reporting specific gravities, since the density of the substance and of water change with temperature. Specific gravities are often reported relative to water at 4°C; at that temperature, water has a density of 1.00000 g/mL and the specific gravity of a substance is equal to its density in g/mL. Specific Volume. Compare With Density. The volume of a unit mass of substance. For example, the specific volume of water at 4°C is 1.00000 mL/g. Specific volume is the reciprocal of density. Standard Deviation. (S,BESD,) The standard deviation is a statistical measure of precision. The best estimate of the standard deviation s for small data sets is calculated using where xi is the measurement from the i-th run, x-bar is the mean of all the measurements, and Nis the number of measurements. For very large data sets, the standard deviation is the root-mean-square deviation from the true mean, and is usually written as to distinguish it from the best estimate standard deviation s used for small data sets. Systematic Error. Determinate Error. Compare With Random Error, Gross Error And Mistake. Systematic errors have an identifiable cause and affect the accuracy of results. Trueness. Compare With Accuracy. Trueness is the closeness of an average measurement to a "true" value, while accuracy is the the closeness of a single measurement to the true value. Unit. A standard for comparison in measurements. For example, the meter is a standard length which may be compared to any object to describe its length. Weight. (W) Compare With Mass. Weight is the force exerted by an object in a gravitational field. The weight of an object (W) arises from its mass (m): W = mg where g is the acceleration due to gravity (about 9.8 m/s2 on Earth).