Here we will cover measurement and why it’s so important to have correct measurements in experimental sciences and chemistry overall. A measurement is a quantity that possesses both a number and a unit. For example, a measurement of your height might be 5 ft, which contains a number, 5, and a unit, ft. In chemistry, scientists will often work with very large numbers so, to avoid writing the full length of the number, they will use scientific notation in which a given number is written as the product of a coefficient and 10 raised to a power. For example, 405,000,000,000 is 4.05 x 1011.
In chemistry, it is essential that all calculations and measurements are precise so that experimental error can be kept to a minimum and so that the experiment can be reproduced accurately. Accuracy is the measure of how close the measurement is to the true value. Precision is the measure of how close a series of measurements are to one another. In order to evaluate the accuracy of a measurement, the measured value should be compared to the true value and, to evaluate precision, two or more repeated measurements should be compared.
When measurements are inaccurate, there is a difference between the accepted value, which is the correct value based on references, and the experimental value, which is the value measured in the lab. The difference is called the error. This error can be negative or positive.
Error = experimental value – accepted value
Error can also be calculated as a percentage as the relative error, or percent error.
Percent Error = (|error|/accepted value)*100%
Scientists typically use units from the International System of Measurement (SI). A universal system is key when sharing findings with people in other parts of the world. It also makes for easy conversions. There are five SI units used by chemists and they are the meter, the kilogram, the kelvin, the second, and the mole. The meter is the basic unit of length. Depending on the size of the object being measured, variations of the meter can be used. For smaller objects, it is more practical to use centimeters or millimeters, and for large objects, it may be more appropriate to use meters or kilometers. To measure volume, the liter is most commonly used. For smaller units, it may be better to use milliliters or microliters. When measuring mass, kilograms and grams are used. Energy, which is the capacity to do work or produce heat, is measured in joules(J). A calorie(cal) is the quantity of heat that raises the temperature of 1 g of pure water by 1 degree Celsius. To convert between the two, just remember:
1 J = 0.2390 cal and 1 cal = 4.184 J
Temperature is measured in Celsius(C) or Kelvin(K). While the Celsius scale sets water’s freezing point at 0 degrees, the Kelvin cycle considers 0 K absolute 0. In Celsius, this is -273.15 degrees! When converting from Celsius to kelvin, just add 273.15 to the number of degrees in Celsius. To convert from Kelvin to Celsius, subtract 273.25 from the number of degrees in Kelvin. this works since 0 degrees in Celsius is equal to 273.15 K in Kelvin.
It’s important to know how to convert between units in chemistry to be able to express quantities in different ways. For example when you convert between 4 quarters and a dollar. Whenever there are two measurements that are the same, their ratios will equal 1.
(1 dollar)/(1 dollar) = (4 quarters)/(1 dollar) = 1
When converting between equivalent units, the conversion factor will be used. The conversion factor is the number used to change one set of units to another, by multiplying or dividing. In this case, the conversion factor is 1 dollar. It shows the ratio between quarters and dollars. For every dollar, there is an equivalent of 4 quarters.
Often units are put together in order to show the relationship between two different quantities. An example of this is density which is the relationship between the mass of an object and its volume.
Density = mass/volume
The units for mass are grams and the units for volume are lengths of measurement cubed. For example, the units for an object's density may be 10 g/cm3 . This unit shows the relationship between mass and volume when determining density. Density is an intensive property since it relies on the composition of the substance and not its size. It is the mass per unit of volume.
Density can also be changed with temperature. Mass remains the same but volume increases as temperature increases. This means that density will also increase. Density decreases as temperature increases which makes sense if you recall that density inversely varies with volume.
References:
Wilbraham, Antony C. Prentice Hall Chemistry. Prentice Hall, 2008.
Photo Credit: Myers, Oldham & Tocci, Holt Chemistry, p. 12
Written by: Mahathi Somula
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