Science involved activities which needs to be measured by using measuring instrument like platform balance, graduated cylinder, ruler, etc. Using this instruments have corresponding units to be used. Properties of matter also are mostly quantitative and they are associated also with numbers. The properties can be meaningless without using units. Like for example the density of water, when we just say 1, it doesn't have meaning; but when we say 1 g/mL, that is now understood that we are talking about density because of the attached unit. Can you now see the difference?
In this post, I will be focusing on the metric system of measurement. The metric system was first developed in France during the late 18th century, and is used by most countries throughout the world.
SI BASE UNITS
In 1960, there was an agreement specifying the use of metric units in scientific measurement. These units are called the SI Units , which means International System of Units. ( taken from the French Systeme International d' Unites). The SI system has seven SI Base Units and this is also the basis of some derived units. See table below for SI Base Units:
Mass and Weight
Mass and weight are often used interchangeably, but the two are different quantities. Mass is a measure of the quantity of matter in an object, while weight is the force that gravity exerts in an object. Weight is dependent on the location of an object while the mass does not. The mass of the apple is constant and does not depend on its location but its weight will be different depending on its location. Example, apple on the surface of the moon would weigh only one-sixth what it does on earth, simply because on the moon, it has less gravity.
The SI base unit of mass is kilogram (kg), which is equal to 1000 g. In the laboratory, the most convenient way to use unit is g, because we measure small quantities of matter. That's is why in calculation, once we need value in kg we do conversion of units using the prefixes in the table above.
Temperature
Temperature is another SI base quantity which has Kelvin as the SI unit. It is a measure of hotness and coldness of a body. It can be measured using thermometer. There are three temperature scales, there are degree Celsius (oC), degree Fahrenheit (oF), and the Kelvin (K) scale.
Below are the comparison of the three scales:
The freezing point of water in degree Celsius is 0oC, which is equivalent to 273.15 K in Kelvin, and the boiling point of water in degree Celsius is 100oC which is equivalent to 373.15 K in Kelvin. Absolute zero in Kelvin is equivalent to -273.15oC. Both Celsius and Kelvin scales have the same magnitude. As such, we can convert one scale to another by using the formulas below:
K = oC + 273
oC = K - 273
In degree Fahrenheit and degree Celsius comparison on the other hand, is different. That is why the formulas are different when converting one from the other. The size of Fahrenheit scale is only 5/9 of a degree on the Celsius scale. Therefore the formula are as follows:
oC = 5/9 (oF - 32)
oF = 9/5 (oC) + 32
Sample Exercise:
If a weather forecaster predicts that the temperature for the day will reach 31oC, what is the predicted temperature in a) in K, and b) in oF?
Solution:
a) K = oC + 273
= 31 + 273
= 304 K
b) oF = 9/5(oC) + 32
= 9/5(31) + 32
= 56 + 32
= 88 oF
DERIVED SI UNITS
Derived units are based from SI base units. In chemistry we usually used volume, density, pressure, force, energy, etc, as examples of derived units.
Volume
Volume is length (m) cubed. Thus the SI Derived unit for volume is cubic meter or m3. Sometimes, smaller units such as cm3 and dm3 are also used in chemistry.
1 cm3 = 1 x 10-6 m3
1dm3 = 1x 10-3 m3
Another common unit for volume is liter (L), which is a volume occupied by one cubic decimeter (dm3). One liter (L) is equal to 1000 ml or 1000 cubic centimeters. One cubic centimeter is equal to 1 milliliter (mL).
1L = 1000 mL
= 1000 cm3
= 1 dm3
Density
Density is the amount of mass in a unit volume of a substance.
The SI derived unit for density is kilogram per cubic meter (kg/m3). This is large unit for chemical calculations and so gram per cubic centimeter (g/cm3) or gram per milliliter (g/mL) are used instead.
Below is a table for some densities of some selected substances at 25oC:
Sample Problem:
a) Gold is a precious metal that is chemically unreactive. A piece of gold with a mass of 301 g has a volume of 15.6 mL. What is the density of gold?
b) Calculate the volume of 65.0 g of the liquid methanol if its density is 0.791 g/mL.
c) A piece of platinum metal with a density of 21.5 g/mL has a volume of 4.49 mL.
Solution:
a) Density = mass / volume
= 301g / 15.6 mL
= 19.3 g/mL
b) Volume = mass/density
= 65.0g / 0.791 g/mL
= 82. 2 mL
c) Mass = volume x density
= 4.49 mL x 21.5 g/mL
= 96.5 g
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