# 5.4 Writing formulae

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## 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

1. A molecule of water contains the elements hydrogen and oxygen .

2. 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.

3. 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:

$\mathrm{H}{}_{2}\mathrm{O}$ .

Write the chemical formula for magnesium oxide

1. A molecule of magnesium oxide contains the elements magnesium and oxygen .

2. 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.

3. The chemical formula for magnesium oxide is therefore:

$\mathrm{MgO}$

Write the chemical formula for copper (II) chloride.

1. A molecule of copper (II) chloride contains the elements copper and chlorine .

2. 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.

3. The chemical formula for copper (II) chloride is therefore:

$\mathrm{CuCl}{}_{2}$

## 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 $\mathrm{NaCl}$ 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 $\mathrm{MgO}$ , 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]

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

What is price elasticity of demand and its degrees. also explain factors determing price elasticity of demand?
Price elasticity of demand (PED) is use to measure the degree of responsiveness of Quantity demanded for a given change on price of the good itself, certis paribus. The formula for PED = percentage change in quantity demanded/ percentage change in price of good A
GOH
its is necessarily negative due to the inverse relationship between price and Quantity demanded. since PED carries a negative sign most of the time, we will usually the absolute value of PED by dropping the negative sign.
GOH
PED > 1 means that the demand of the good is price elasticity and for a given increase in price there will be a more then proportionate decrease in quantity demanded.
GOH
PED < 1 means that the demand of the good is price inelasticity and for a given increase in price there will be a less then proportionate decrease in quantity demanded.
GOH
The factors that affects PES are: Avaliablilty of close substitutes, proportion of income spent on the good, Degree of necessity, Addiction and Time.
GOH
Calculate price elasticity of demand and comment on the shape of the demand curve of a good ,when its price rises by 20 percentage, quantity demanded falls from 150 units to 120 units.
5 %fall in price of good x leads to a 10 % rise in its quantity demanded. A 20 % rise in price of good y leads to do a 10 % fall in its quantity demanded. calculate price elasticity of demand of good x and good y. Out of the two goods which one is more elastic.
Helen
what is labor
labor is any physical or mental effort that helps in the production of goods and services
Kwabena
what is profit maximizing level of out put for above hypothetical firm TC = Q3 - 21Q2 + 600 + 1800 P = 600 MC = 3Q2 - 42Q + 600
consider two goods X and Y. When the price of Y changes from 10 to 20. The quantity demanded of X changes from 40 to 35. Calculate cross elasticity of demand for X.
Sosna
sorry it the mistake answer it is question
Sosna
consider two goods X and Y. When the price of Y changes from 10 to 20. The quantity demanded of X changes from 40 to 35. Calculate cross elasticity of demand for X.
Sosna
The formula for calculation income elasticity of demand is the percent change in quantity demanded divided by the percent change in income.
Sosna
what is labor productivity
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what are some of the difference between monopoly and perfect competition market
n a perfectly competitive market, price equals marginal cost and firms earn an economic profit of zero. In a monopoly, the price is set above marginal cost and the firm earns a positive economic profit. Perfect competition produces an equilibrium in which the price and quantity of a good is economic
Naima
what are some characteristics of monopoly market
explicit cost is seen as a total experiences in the business or the salary (wages) that a firm pay to employee.
what is price elasticity
Fosua
...
krishna
it is the degree of responsiveness to a percentage change in the price of the commodity
Obeng
economics is known to be the field
what is monopoly
what is taxation
Peter
is the compulsory transfer of wealth from the private sector to the public sector
Jonna
why do monopoly make excess profit in both long run and short run
because monopoly have no competitor on the market and they are price makers,therefore,they can easily increase the princes and produce small quantity of goods but still consumers will still buy....
Kennedy
how to identify a perfect market graph
what is the investment
jimmy
investment is a money u used to the business
Mohamed
investment is the purchase of good that are not consumed today but are used in the future to create wealth.
Amina
investment is the good that are not consumed
Fosua
What is supply
Fosua
Supply represents how much the market can offer.
Yusif
it is the quantity of commodity producers produces at the market
Obeng
what is the effect of scarce resources on producers
explain how government taxes and government producer subsidies affect supply
Chanda
what is economic
what are the type of economic
Charles
macroeconomics,microeconomics,positive economics and negative economics
what are the factors of production
process of production
Mutia
Basically factors of production are four (4) namely: 1. Entrepreneur 2. Capital 3. Labour and; 4. Land but there has been a new argument to include an addition one to the the numbers to 5 which is "Technology"
Elisha
what is land as a factor of production
what is Economic
Abu
economics is how individuals bussiness and governments make the best decisions to get what they want and how these choices interact in the market
Nandisha
Economics as a social science, which studies human behaviour as a relationship between ends and scarce means, which have alternative uses.
Yhaar
Economics is a science which study human behaviour as a relationship between ends and scarce means
John
Economics is a social sciences which studies human behavior as a relationship between ends and scarce mean, which have alternative uses.....
Pintu
A soccer field is a rectangle 130 meters wide and 110 meters long. The coach asks players to run from one corner to the other corner diagonally across. What is that distance, to the nearest tenths place.
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why surface tension is zero at critical temperature
Shanjida
I think if critical temperature denote high temperature then a liquid stats boils that time the water stats to evaporate so some moles of h2o to up and due to high temp the bonding break they have low density so it can be a reason
s.
Need to simplify the expresin. 3/7 (x+y)-1/7 (x-1)=
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The fundamental frequency of a sonometer wire streached by a load of relative density 's'are n¹ and n² when the load is in air and completly immersed in water respectively then the lation n²/na is
Properties of longitudinal waves