15.1 Stoichiometry and composition (Page 29/5)

Siyavula textbooks: grade 10 Page 1 / 5

The composition of substances

The empirical formula of a chemical compound is a simple expression of the relative number of each type of atom in that compound. In contrast, the molecular formula of a chemical compound gives the actual number of atoms of each element found in a molecule of that compound.

Empirical formula

The empirical formula of a chemical compound gives the relative number of each type of atom in that compound.

Molecular formula

The molecular formula of a chemical compound gives the exact number of atoms of each element in one molecule of that compound.

The compound ethanoic acid for example, has the molecular formula $CH_{3} COOH$ or simply $C_{2} H_{4} O_{2}$ . In one molecule of this acid, there are two carbon atoms, four hydrogen atoms and two oxygen atoms. The ratio of atoms in the compound is 2:4:2, which can be simplified to 1:2:1. Therefore, the empirical formula for this compound is $CH_{2} O$ . The empirical formula contains the smallest whole number ratio of the elements that make up a compound.

Knowing either the empirical or molecular formula of a compound, can help to determine its composition in more detail. The opposite is also true. Knowing the composition of a substance can help you to determine its formula. There are four different types of composition problems that you might come across:

Problems where you will be given the formula of the substance and asked to calculate the percentage by mass of each element in the substance.
Problems where you will be given the percentage composition and asked to calculate the formula.
Problems where you will be given the products of a chemical reaction and asked to calculate the formula of one of the reactants. These are often referred to as combustion analysis problems.
Problems where you will be asked to find number of moles of waters of crystallisation.

Calculate the percentage that each element contributes to the overall mass of sulphuric acid ( $H_{2} {SO}_{4}$ ).

Write down the relative atomic mass of each element in the compound
$Hydrogen = 1,008 \times 2 = 2,016 u$

$Sulphur = 32,07 u$

$Oxygen = 4 \times 16 = 64 u$
Calculate the molecular mass of sulphuric acid
Use the calculations in the previous step to calculate the molecular mass of sulphuric acid.

$Mass = 2, 016 + 32, 07 + 64 = 98, 09 u$
Convert the mass of each element to a percentage of the total mass of the compound
Use the equation:

$Percentage by mass = \frac{atomic mass}{molecular mass of H_{2} SO_{4}} \times 100 %$

Hydrogen

$\frac{2, 016}{98, 09} \times 100 % = 2, 06 %$

Sulphur

$\frac{32, 07}{98, 09} \times 100 % = 32, 69 %$

Oxygen

$\frac{64}{98, 09} \times 100 % = 65, 25 %$

(You should check at the end that these percentages add up to 100%!)

In other words, in one molecule of sulphuric acid, hydrogen makes up 2,06% of the mass of the compound, sulphur makes up 32,69% and oxygen makes up 65,25%.

A compound contains 52.2% carbon ( $C$ ), 13.0% hydrogen ( $H$ ) and 34.8% oxygen ( $O$ ). Determine its empirical formula.

If we assume that we have $100 g$ of this substance, then we can convert each element percentage into a mass in grams
Carbon $= 52,2 g$ , hydrogen $= 13,0 g$ and oxygen $= 34,8 g$
Convert the mass of each element into number of moles
$n = \frac{m}{M}$

Therefore,

$n (Carbon) = \frac{52, 2}{12, 01} = 4, 35 mol$

$n (Hydrogen) = \frac{13, 0}{1, 008} = 12, 90 mol$

$n (Oxygen) = \frac{34, 8}{16} = 2, 18 mol$
Convert these numbers to the simplest mole ratio by dividing by the smallest number of moles
In this case, the smallest number of moles is 2.18. Therefore...

Carbon

$\frac{4, 35}{2, 18} = 2$

Hydrogen

$\frac{12, 90}{2, 18} = 6$

Oxygen

$\frac{2, 18}{2, 18} = 1$

Therefore the empirical formula of this substance is: $C_{2} H_{6} O$ . Do you recognise this compound?

Questions & Answers

if three forces F1.f2 .f3 act at a point on a Cartesian plane in the daigram .....so if the question says write down the x and y components ..... I really don't understand

Syamthanda Reply

hey , can you please explain oxidation reaction & redox ?

Boitumelo Reply

hey , can you please explain oxidation reaction and redox ?

Boitumelo

for grade 12 or grade 11?

Sibulele

the value of V1 and V2

Tumelo Reply

advantages of electrons in a circuit

Rethabile Reply

we're do you find electromagnetism past papers

Ntombifuthi

what a normal force

Tholulwazi Reply

it is the force or component of the force that the surface exert on an object incontact with it and which acts perpendicular to the surface

Sihle

what is physics?

Petrus Reply

what is the half reaction of Potassium and chlorine

Anna Reply

how to calculate coefficient of static friction

Lisa Reply

how to calculate static friction

Lisa

How to calculate a current

Tumelo

how to calculate the magnitude of horizontal component of the applied force

Mogano

How to calculate force

Monambi

a structure of a thermocouple used to measure inner temperature

Anna Reply

a fixed gas of a mass is held at standard pressure temperature of 15 degrees Celsius .Calculate the temperature of the gas in Celsius if the pressure is changed to 2×10 to the power 4

Amahle Reply

How is energy being used in bonding?

Raymond Reply

what is acceleration

Syamthanda Reply

a rate of change in velocity of an object whith respect to time

Khuthadzo

how can we find the moment of torque of a circular object

Kidist

Acceleration is a rate of change in velocity.

Justice

t =r×f

Khuthadzo

how to calculate tension by substitution

Precious Reply

Shongi

Leago

use fnet method. how many obects are being calculated ?

Khuthadzo

khuthadzo hii

Hulisani

how to calculate acceleration and tension force

Lungile Reply

you use Fnet equals ma , newtoms second law formula

Masego

please help me with vectors in two dimensions

Mulaudzi Reply

how to calculate normal force

Mulaudzi

Got questions? Join the online conversation and get instant answers!

Jobilize.com Reply

<< Chapter < Page Page > Chapter >>

Read also:

Get Jobilize Job Search Mobile App in your pocket Now!

100% Free Mobile Applications
Receive real-time job alerts and never miss the right job again

Source: OpenStax, Siyavula textbooks: grade 10 physical science [caps]. OpenStax CNX. Sep 30, 2011 Download for free at http://cnx.org/content/col11305/1.7

Google Play and the Google Play logo are trademarks of Google Inc.

Notification Switch

Would you like to follow the 'Siyavula textbooks: grade 10 physical science [caps]' conversation and receive update notifications?

Ask

	21 AP Key Terms 21 Lymphatic Immune System By OpenStax Start Key Terms
	Worldport Outside By Rachel Carlisle Start Quiz
©flickr: Jovial	Video Mid Term By Dewey Compton Start Exam
	31 Biology 31 Soil and Plant Nutrition MCQ By OpenStax Start Quiz
	5 AP 05 Integumentary System MCQ By OpenStax Start Quiz
	MAPEH Test G9 (Physical Education) By Angelica Lito Start Quiz
	MAPEH TEST By Edgar Delgado Start Test
	6 Microbiology Midterm Practice By Madison Christian Start Test
©flickr: Tim	Hazard Quiz By Royalle Moore Start Quiz
	3 Microeconomics 03 Demand Supply By OpenStax Start Flashcards