5.4 Writing formulae (Page 4/2)

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Writing chemical formulae

The formulae of covalent compounds

To work out the formulae of covalent compounds, we need to use the valency of the atoms in the compound. This is because the valency tells us how many bonds each atom can form. This in turn can help to work out how many atoms of each element are in the compound, and therefore what its formula is. The following are some examples where this information is used to write the chemical formula of a compound.

Write the chemical formula for water

Write down the elements that make up the compound.
A molecule of water contains the elements hydrogen and oxygen .
Determine the valency of each element
The valency of hydrogen is 1 and the valency of oxygen is 2. This means that oxygen can form two bonds with other elements and each of the hydrogen atoms can form one.
Write the chemical formula
Using the valencies of hydrogen and oxygen, we know that in a single water molecule, two hydrogen atoms will combine with one oxygen atom. The chemical formula for water is therefore:

$H_{2} O$ .

Write the chemical formula for magnesium oxide

Write down the elements that make up the compound.
A molecule of magnesium oxide contains the elements magnesium and oxygen .
Determine the valency of each element
The valency of magnesium is 2, while the valency of oxygen is also 2. In a molecule of magnesium oxide, one atom of magnesium will combine with one atomof oxygen.
Write the chemical formula
The chemical formula for magnesium oxide is therefore:

$MgO$

Write the chemical formula for copper (II) chloride.

Write down the elements that make up the compound.
A molecule of copper (II) chloride contains the elements copper and chlorine .
Determine the valency of each element
The valency of copper is 2, while the valency of chlorine is 1. In a molecule of copper (II) chloride, two atoms of chlorine will combine with one atom of copper.
Write the chemical formula
The chemical formula for copper (II) chloride is therefore:

$CuCl_{2}$

The formulae of ionic compounds

The overall charge of an ionic compound will always be zero and so the negative and positive charge must be the same size. We can use this information to work out what the chemical formula of an ionic compound is if we know the charge on the individual ions. In the case of $NaCl$ for example, the charge on the sodium is $+ 1$ and the charge on the chlorine is $- 1$ . The charges balance ( $+ 1 - 1 = 0$ ) and therefore the ionic compound is neutral. In $MgO$ , magnesium has a charge of $+ 2$ and oxygen has a charge of $- 2$ . Again, the charges balance and the compound is neutral. Positive ions are called cations and negative ions are called anions .

Some ions are made up of groups of atoms, and these are called compound ions . It is a good idea to learn the compound ions that are shown in [link]

Table showing common compound ions and their formulae

Name of compound ion	formula	Name of compound ion	formula
Carbonate	$CO_{3}^{2 -}$	Nitrite	${NO}_{2}^{-}$
Sulphate	${SO}_{4}^{2 -}$	Hydrogen sulphite	${HSO}_{3}^{-}$
Hydroxide	${OH}^{-}$	Hydrogen sulphate	${HSO}_{4}^{-}$
Ammonium	${NH}_{4}^{+}$	Dihydrogen phosphate	$H_{2} {PO}_{4}^{-}$
Nitrate	${NO}_{3}^{-}$	Hypochlorite	${ClO}^{-}$
Hydrogen carbonate	${HCO}_{3}^{-}$	Acetate (ethanoate)	${CH}_{3} {COO}^{-}$
Phosphate	${PO}_{4}^{3 -}$	Oxalate	$C_{2} O_{4}^{2 -}$
Chlorate	${ClO}_{3}^{-}$	Oxide	$O^{2 -}$
Cyanide	${CN}^{-}$	Peroxide	$O_{2}^{2 -}$
Chromate	${CrO}_{4}^{2 -}$	Sulphide	$S^{2 -}$
Permanganate	${MnO}_{4}^{-}$	Sulphite	${SO}_{3}^{2 -}$
Thiosulphate	$S_{2} O_{3}^{2 -}$	Manganate	${MnO}_{4}^{2 -}$
Phosphide	$P^{3 -}$	Hydrogen phosphate	${HPO}_{4}^{3 -}$

Questions & Answers

A golfer on a fairway is 70 m away from the green, which sits below the level of the fairway by 20 m. If the golfer hits the ball at an angle of 40° with an initial speed of 20 m/s, how close to the green does she come?

Aislinn Reply

tijani

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John Reply

what is physics

Siyaka Reply

A mouse of mass 200 g falls 100 m down a vertical mine shaft and lands at the bottom with a speed of 8.0 m/s. During its fall, how much work is done on the mouse by air resistance

Jude Reply

Can you compute that for me. Ty

Jude

what is the dimension formula of energy?

David Reply

what is viscosity?

David

what is inorganic

emma Reply

what is chemistry

Youesf Reply

what is inorganic

emma

Chemistry is a branch of science that deals with the study of matter,it composition,it structure and the changes it undergoes

Adjei

please, I'm a physics student and I need help in physics

Adjanou

chemistry could also be understood like the sexual attraction/repulsion of the male and female elements. the reaction varies depending on the energy differences of each given gender. + masculine -female.

Pedro

A ball is thrown straight up.it passes a 2.0m high window 7.50 m off the ground on it path up and takes 1.30 s to go past the window.what was the ball initial velocity

Krampah Reply

2. A sled plus passenger with total mass 50 kg is pulled 20 m across the snow (0.20) at constant velocity by a force directed 25° above the horizontal. Calculate (a) the work of the applied force, (b) the work of friction, and (c) the total work.

Sahid Reply

you have been hired as an espert witness in a court case involving an automobile accident. the accident involved car A of mass 1500kg which crashed into stationary car B of mass 1100kg. the driver of car A applied his brakes 15 m before he skidded and crashed into car B. after the collision, car A s

Samuel Reply

can someone explain to me, an ignorant high school student, why the trend of the graph doesn't follow the fact that the higher frequency a sound wave is, the more power it is, hence, making me think the phons output would follow this general trend?

Joseph Reply

Nevermind i just realied that the graph is the phons output for a person with normal hearing and not just the phons output of the sound waves power, I should read the entire thing next time

Joseph

Follow up question, does anyone know where I can find a graph that accuretly depicts the actual relative "power" output of sound over its frequency instead of just humans hearing

Joseph

"Generation of electrical energy from sound energy | IEEE Conference Publication | IEEE Xplore" ***ieeexplore.ieee.org/document/7150687?reload=true

Ryan

what's motion

Maurice Reply

what are the types of wave

Maurice

answer

Magreth

progressive wave

Magreth

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Mujahid

A string is 3.00 m long with a mass of 5.00 g. The string is held taut with a tension of 500.00 N applied to the string. A pulse is sent down the string. How long does it take the pulse to travel the 3.00 m of the string?

yasuo Reply

Who can show me the full solution in this problem?

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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

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