Whats in a mole?

Overview

In real life situations, pieces of matter are quantified by finding the mass or by counting. Market vendors for example, would rather sell mongo seeds by getting the mass and salted eggs by counting. The choice of quantifying goods is determined by convenience. It is easier to get the mass of rice grains rather than count the grains. It is more convenient to count the number of eggs rather than get their mass. To measure these quantities, mass units such as kilogram or gram, or counting units such as dozen or case are being used.

In the laboratory, chemists measure out a chemical substance and react it with another substance to form the desired quantity of a new product. In this case, chemists want to know the number of atoms, ions, or molecules because these are the ones that react with each other. However, these things are too small and too many to count individually so chemists use a unit called mole to count them by weighing. Like a dozen, a ream, or a case, a mole also represents a certain number of particles. Can you guess how many particles are equal to one mole?

Avogadro's number is the number of particles in one mole of a substance. It is a very large number equal to 6.02 x 1023 particles. So, a mole 23 (mol) of a substance is 6.02 x 10 representative particles of that substance. The representative particles can be atoms, molecules, or formula units. So, one mole of 23 carbon-12 contains 6.02 x 10 atoms for elements molecules for covalent compounds, or formula units for ionic compounds. So, one mole of carbon contains 23 23 6.02 x 10 atoms, one mole of water contains 6.02 x 10 water molecules and a mole of sodium chloride (table salt) contains 6.02 x 10 formula units of sodium 23 chloride. For you to figure out how large Avogadro's number is, try to imagine this, "if you put together 6.02 x 10 23 basketballs, it will be as big as the Earth or if you have 6.02 x 10 23 rice grains, it would cover the land masses of the Earth to a depth of 75 meters." Now that you have realized how big Avogadro's number is, let us try to visualize the number of particles in a given sample.

Percentage Composition of a Compound

Are you interested to know how much of an element is present in a compound? You can answer this question by determining percentage composition. The mass of each element in a compound compared to the entire mass of the compound multiplied by 100 percent is called the percentage composition of the compound. So, the percentage composition of a compound tells you the percentage of the mass made up by each element in a compound.

Let us compare it in a classroom situation. You are 50 in your section with 21 boys and 29 girls. If you will be asked what percent of the class are boys and what percent are girls, how are you going to compute for the answer? If your answer is 42% boys and 58% girls, you got it correctly! Let us have an example for the compound which is so important to all of us, water (H2O). The computation below shows the molar mass of water. If you will be asked to compute for the percentage of oxygen and hydrogen in water, how are you going to do it?

H₂O

10 atom (16.00 g) = 16.00 g 2 H atoms (1.01g) = 2.02 g

18.02 g

Did you answer 89% oxygen and 11% hydrogen? Your answer is correct! To get the percent oxygen, mass of oxygen is divided by the mass of water multiplied by 100%. The same is true with hydrogen.

Knowledge about percentage composition is useful in choosing the best fertilizer that gives higher amount of nitrogen and potassium in the soil. Aside from this, it will help us to be a wise consumer because it is a good basis for choosing the food product we need to eat based on its content. Whenever you buy t-shirt, it is good to check whether it is 100% cotton or just 75% cotton. In terms of product development, percentage composition will indicate the correct amount of substances needed in order to produce the expected product. For example, if a manufacturing company wants to produce resin, it will not be able to do if it does not know the correct percentage of every component needed to form resin.

Summary:

It is important that you remember the following ideas from this module:

Different kinds of materials with the same number of particles have different masses. One mole contains Avogadro's number of particles equal to 6.02 x 10 23

Different substances with the same number of moles have the same number of particles but they have different masses. Molar mass is the mass of one mole of a substance expressed in grams.

The molar mass of a monatomic element like Na, Li, Mg is numerically equal to its atomic mass expressed in grams.

The mass of the substance divided by its molar mass gives the number of moles of the substance.

The number of moles multiplied by Avogadro's number gives the number of particles. Percentage composition of a compound tells you the percentage of the mass made up by each element in a compound.