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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 are perpendicular vectors
Sphelele Reply
how are you
Perpendicular vectors are vectors on the y-axis
Definition of vector?
Mufaro Reply
a physical quantity that has both magnitude and direction e.g force : velocity : displacement etc.
wrong text
Definition of resultant vector
physical quantity that has both magnitude and direction
definition of vector
Ndlezana Reply
a physical quantity that has both a magnitude and a direction
how to determine the magnitude and direction of the resultant force, By CALCULATION Using a scale of ( 10mm = 50N.)?
Sfundo Reply
hi guys I'm new here
Ezile Reply
feel at home 😊
you welcome
Asanda Simamane
why forces are classified as vectors
Sfundo Reply
because it has both size and the direction
you can jus pull or push something and jus tell that you were pulling it in no direction
Tobias Louw I highly appreciate that 😊
happy to help
what is a molecular?
Kamogelo Reply
A molecule is covalently bonded. Meaning is it is a bond between two or more non-metals.
I mean what is a resulted vector
Njabulo Reply
is the vector quantity that has the same effects as 2 or more vectors
It is the combination of two or more vectors
is the vector sum of two or more vectors
what are compartments of vectors
yes junior
hey how do I calculate the unknown force using algebraic calculation
Lizzy Reply
I also need to know that help me please
help me too n calculating it
I need to know more about components of vectors
help me too, i dont know to
what is a vector
what is covalent bonding
Jeanine Reply
is the sharing of atoms between two or more substances
Draw a 100 N force vector acting towards the right
Welcome Reply
Two forces act on an object. The first is a horizontal 50N force. The second is a 40N force acting at an angle to the horizontal 1.1 use a graphical method drawn to 1mm:1N to find the resultan if 40N force is inclined at: 1.1 30° to the horizontal 1.2 60° to the horizontal
A gardener mounts a pot plant on the wall using a metal ring and chain. The ring exerts a horizontal force of 20 N on the pot plant and the weight of the pot plant is 30 N Determine the magnitude of the tension in the chain by calculation and construction
Amelia Reply
what is a head to tail methods?
Karabo Reply
thats simply involves drawing the first vector and then joining the second vector to its tail then join the head of the firstvector
what is magnitude
Mfundo Reply
Magnitude is size
magnitude is the maximum or greater extent of size and the direction of an object, it describes the direction of vectors and scalars.
So I got a question that follows: two forces of 60N and 80N respectively act simultaneously at a point, sketch and determine their resultant by using the tail to head method
Mbuli Reply

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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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