# Moles and molar mass

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## Introduction

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:

$Fe+S\to FeS$

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).

 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 ${10}^{23}$

what are perpendicular vectors
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phelisa
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Perpendicular vectors are vectors on the y-axis
Sphiwe
Definition of vector?
a physical quantity that has both magnitude and direction e.g force : velocity : displacement etc.
Mokgetoa
vector
Abigail
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Abigail
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Definition of resultant vector
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physical quantity that has both magnitude and direction
Amahle
definition of vector
a physical quantity that has both a magnitude and a direction
Abigail
how to determine the magnitude and direction of the resultant force, By CALCULATION Using a scale of ( 10mm = 50N.)?
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because it has both size and the direction
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what is a molecular?
A molecule is covalently bonded. Meaning is it is a bond between two or more non-metals.
Sphiwe
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is the vector quantity that has the same effects as 2 or more vectors
junior
It is the combination of two or more vectors
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is the vector sum of two or more vectors
Precious
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what is a vector
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is the sharing of atoms between two or more substances
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Draw a 100 N force vector acting towards the right
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
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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
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what is magnitude
Magnitude is size
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magnitude is the maximum or greater extent of size and the direction of an object, it describes the direction of vectors and scalars.
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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  By By   By By   By Danielrosenberger