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An equation for a chemical reaction can provide us with a lot of useful information. It tells us what the reactants and the products are in the reaction, and it also tells us the ratio in which the reactants combine to form products. Look at the equation below:

F e + S F e S

In this reaction, every atom of iron (Fe) will react with a single atom of sulfur (S) to form one molecule of iron sulfide (FeS). However, what the equation doesn't tell us, is the quantities or the amount of each substance that is involved. You may for example be given a small sample of iron for the reaction. How will you know how many atoms of iron are in this sample? And how many atoms of sulfur will you need for the reaction to use up all the iron you have? Is there a way of knowing what mass of iron sulfide will be produced at the end of the reaction? These are all very important questions, especially when the reaction is an industrial one, where it is important to know the quantities of reactants that are needed, and the quantity of product that will be formed. This chapter will look at how to quantify the changes that take place in chemical reactions.

The mole

Sometimes it is important to know exactly how many particles (e.g. atoms or molecules) are in a sample of a substance, or what quantity of a substance is needed for a chemical reaction to take place.

You will remember from Grade 10 that the relative atomic mass of an element, describes the mass of an atom of that element relative to the mass of an atom of carbon-12. So the mass of an atom of carbon (relative atomic mass is 12 u) for example, is twelve times greater than the mass of an atom of hydrogen, which has a relative atomic mass of 1 u. How can this information be used to help us to know what mass of each element will be needed if we want to end up with the same number of atoms of carbon and hydrogen?

Let's say for example, that we have a sample of 12g carbon. What mass of hydrogen will contain the same number of atoms as 12 g carbon? We know that each atom of carbon weighs twelve times more than an atom of hydrogen. Surely then, we will only need 1g of hydrogen for the number of atoms in the two samples to be the same? You will notice that the number of particles (in this case, atoms ) in the two substances is the same when the ratio of their sample masses (12g carbon: 1g hydrogen = 12:1) is the same as the ratio of their relative atomic masses (12 u: 1 u = 12:1).

To take this a step further, if you were to weigh out samples of a number of elements so that the mass of the sample was the same as the relative atomic mass of that element, you would find that the number of particles in each sample is 6.023 x 10 23 . These results are shown in [link] below for a number of different elements. So, 24.31 g of magnesium (relative atomic mass = 24.31 u) for example, has the same number of atoms as 40.08 g of calcium (relative atomic mass = 40.08 u).

Table showing the relationship between the sample mass, the relative atomic mass and the number of atoms in a sample, for a number of elements.
Element Relative atomic mass (u) Sample mass (g) Atoms in sample
Hydrogen (H) 1 1 6.023 x 10 23
Carbon (C) 12 12 6.023 x 10 23
Magnesium (Mg) 24.31 24.31 6.023 x 10 23
Sulfur (S) 32.07 32.07 6.023 x 10 23
Calcium (Ca) 40.08 40.08 6.023 x 10 23

Questions & Answers

what is inertia
Snegugu Reply
Inertia is the resistance of an object to any change in its state of motion.
inertia is the tendency of a object in rest to remain in rest and an object in motion to remain in motion
what is a normal force
Mandisa Reply
It is a force exerted by the surface on an object in contact with it and it always perpendicular to the surface
A normal force it's a force that perpendicular to any surface
It's an action - reaction force of gravity
how to draw a rough sketch
Believe Reply
what is a vector
it's a physical quantity that has magnitude and direction
Ways of calculating tension
Billy Reply
You need to first find the acceleration
how to calculate a vector
Hazel Reply
How to calculate a vector
how do we calculate acceleration using kinetic friction, applied force and an angle
lehlohonolo Reply
using Fnet = ma. Fnet can be substituted by (Facos(theta) - fk). this Fnet substitution depends on the specifics of the scenario andthe free body diagram
what is a bond?
Xabiso Reply
experiments on this lesson
Anoma Reply
How to calculate tension
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Kavitha Reply
What is the stoichiometry
How would the force of gravity on curiosity change on planet with half the mass and three times the radius of the moon
Amanda Reply
how to calculate forces of different direction?
Lerato Reply
What is atoms
siyanela Reply
how to draw a free body diagram
Mickey Reply
When calculating acceleration in a ticket timer experiment, which time do we use?
Sarah Reply
The time elapsed between the dots you are using. That will be the amount of intervals × the period of the ticker timer.

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Source:  OpenStax, Siyavula textbooks: grade 11 physical science. OpenStax CNX. Jul 29, 2011 Download for free at http://cnx.org/content/col11241/1.2
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