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Formula If you don’t understand this section you will not understand chemistry, get help if required. Rule 1 For most elements the correct ‘formula’ is the chemical symbol, but there are a few elements that exist as diatomic elements. 1. Answer the following: What is a diatomic element? Name the 7 diatomic elements. 2. Write the chemical formula for the following elements. a. sodium b. lithium c. potassium d. calcium e. hydrogen f. chlorine g. helium h. nitrogen i. copper j. oxygen Rule 2 Calculating formulae for two element compounds using prefixes When using prefixes to obtain formula there is no simplifying required. Copy and complete the following table Prefix Meaning Compound Formula Mono 1 Carbon monoxide CO Di Sulphur dioxide Tri Sulphur trioxide tetra Carbon tetrachloride Penta Divanadium pentoxide Hexa Uranium hexafluoride 3. Now copy and complete the following examples Name of compound formula Name of compound a) nitrogen monoxide e) tetraarsenic hexaoxide b) diphosphorus trioxide f) nitrogen dioxide c) silicon tetrachloride g) disulphur dichloride d) boron trifluoride formula Rule 3 – For compounds with names ending in ‘ide’ (2 elements only) and no prefixes you have to use the valencies to work out the formula. The valency of an element can be worked out from its position in the periodic table. Group number 1 2 3 4 5 6 7 Valency 1 2 3 4 3 2 1 Use the 5 step method Step 1 – write down the element symbols Step 2 – write down the valency of each element Step 3 – cross over the valency of each element Step 4 – Simplify Step 5 – Write down the formula. 4. Now write out the chemical formulae for the following 2 element compounds a. Boron Fluoride i. Aluminium oxide b. Carbon sulphide j. Magnesium phosphorus c. Calcium iodide k. Hydrogen sulphide d. Hydrogen fluoride l. Calcium nitride e. Sodium sulphide m. Carbon chloride f. Lithium nitride n. Barium oxide g. Tin bromide o. Potassium oxide h. Strontium bromide p. Calcium hydroxide. Rule 4 - Simple Formulae for compounds containing group ions Page 4 of your data booklet gives a list of group ions that contain more than one kind of atom such as the ammonium ion, NH4+. This ion consists of a group of 5 atoms, one nitrogen and 4 hydrogen atoms, with the whole group having a single positive charge. The valency of a group ion is the same as the number of charges it has. For example, the ammonium ion NH4+, with 1 positive charge, has a valency of one. Example 1 Work out the simple chemical formula for ammonium chloride Step 1 symbols NH4 Cl Step 2 ion charges 1 1 Step 3 cross over charges (NH4)1 Cl1 Step 4 cancel out any common factor (NH4)1 Cl1 Step 5 omit ‘1’ if present NH4Cl Example 2 Work out the simple chemical formula for ammonium carbonate Step 1 symbols NH4 CO3 Step 2 ion charges 1 2 Step 3 cross over charges (NH4)2 (CO3)1 Step 4 cancel out any common factor (NH4)2 CO3 Step 5 omit ‘1’ if present (NH4)2CO3 5. Write simple chemical formulae for the following a) Lithium hydroxide j) Beryllium chromate b) Potassium carbonate k) Sodium dichromate c) Calcium sulphate l) Ammonium bromide d) Strontium carbonate m) Lithium hydrogensulphite e) Potassium chromate n) Potassium sulphate f) Sodium sulphite o) Aluminium phosphate g) Lithium hydrogencarbonate p) Ammonium nitrite h) Sodium nitrate q) Magnesium sulphite i) Potassium phosphate r) Sodium carbonate. Rule 5 - Simple formulae for compounds using Roman Numerals I = 1 V = 5 II = 2 VI = 6 III = 3 VII = 7 IV = 4 Some elements such as the transition metals, can have more than one valency, for example iron can have a valency of 2 or 3. Chemists use Roman numerals to show which valency is being used. Example 1 Work out the simple chemical formula for iron (II) oxide. Step 1 symbols Fe O Step 2 valency 2 2 Step 3 cross over valencies Fe2 O2 Step 4 cancel out any common factor Fe1 O1 Step 5 omit ‘1’ if present FeO. Example 2 Work out the simple chemical formula for iron (III) oxide. Step 1 symbols Fe O Step 2 valency 3 2 Step 3 cross over valencies Fe2 O3 Step 4 cancel out any common factor Fe2 O3 Step 5 omit ‘1’ if present Fe2O3. 6. Write simple chemical formulae for the following a) Iron (II) chloride h) Silver (I) iodide b) Nickel (II) sulphide i) Bismuth (III) sulphide c) Manganese (IV) oxide j) Silver (II) fluoride d) Tin (IV) chloride k) Copper (I) oxide e) Lead (II) iodide l) Uranium (VI) fluoride f) Lead (II) oxide m) Manganese (VII) oxide g) Antimony (V) oxide n) Bismuth (III) hydride Rule 6 - Simple formulae for compounds using brackets If more than one of the same group ion is present in a chemical formula, then brackets are placed around that ion, followed by a number to indicate how many of those ions are present. For example the formula Ca(OH)2, indicates that, in calcium hydroxide, there are 2 hydroxide ions, OH-, for every calcium ion, Ca2+ Example Work out the simple chemical formula for calcium hydroxide. Step 1 symbols Ca OH Step 2 ion charge 2 1 Step 3 cross over charge Ca1 (OH)2 Step 4 cancel out any common factor Ca1 (OH)2 Step 5 omit ‘1’ if present Ca(OH)2. 7. Write simple chemical formulae for the following a) Magnesium hydroxide h) Ammonium sulphite b) Barium phosphate i) Barium hydrogensulphate c) Beryllium nitrate j) Aluminium hydroxide d) Calcium hydrogencarbonate k) Aluminium sulphate e) Ammonium phosphate l) Strontium nitrite f) Magnesium nitrate m) Calcium hydrogensulphite g) Calcium nitrite n) Ammonium carbonate. Word Equations In all chemical reactions one or more new substances are formed, the initial substances in the reaction are called the reactants. The substances formed in the reaction are called the products. Word equations can be written for any chemical reaction that shows the names of the reactants and the products. In all chemical reactions, including word equations ‘+’ means ‘and’ and ‘ ‘ means ‘reacts to produce’. Example - methane is burned in oxygen to produce carbon dioxide and water. Methane CH4 8. + oxygen carbon dioxide + O2 CO2 water H 2O Write word equations for the following reactions and then underneath write the chemical formula for each compound. a) Sodium reacting with chlorine to give sodium chloride b) Hydrogen reacting with bromine to give hydrogen bromide c) Magnesium reacting with oxygen to give magnesium oxide d) Calcium carbonate decomposing to give calcium oxide and carbon dioxide e) Potassium hydroxide reacting with carbon dioxide to give potassium carbonate and water. f) Lithium oxide reacting with water to give lithium hydroxide. g) Barium nitrate and sodium sulphate reacting to produce barium sulphate and sodium nitrate h) Zinc and copper (II) chloride reacting to produce copper and zinc (II) chloride. Balancing Chemical Equations During a chemical reaction atoms can be neither created nor destroyed. All of the atoms present in the reactants are still present in the products. This means that to write out a chemical equation properly we have to have the same number of each type of atom on each side of the equation. This is called balancing equations Example In the complete combustion of methane, methane and oxygen react to produce carbon dioxide and water. This can be written as shown CH4 Count the number Left: H atoms C atoms O atoms + O2 CO2 + H 2O of atoms on each side of the equation Right: 4 H atoms 1 C atoms 2 O atoms 2 1 3 Now you need to work out how to make each side balance. If we look at the hydrogen atoms first, the only product produced from hydrogen is water. The 4 hydrogen atoms in the methane molecule will produce 2 water molecules CH4 + O2 CO2 + 2H2O This means that there will be 1 more oxygen on the right hand side from the extra molecule of water Left: Right: H atoms 4 H atoms 4 C atoms 1 C atoms 1 O atoms 2 O atoms 4 We must have used 2 molecules of oxygen to burn 1 molecule of methane CH4 Left: H atoms C atoms O atoms + 4 1 4 2O2 CO2 Right: + 2H2O H atoms C atoms O atoms 4 1 4 Example 2 Balance the equation H2 + O2 H 2O Count the number of atoms on each side of the equation Left: Right: H atoms 2 H atoms O atoms 2 O atoms 2 1 Now you need to work out how to make each side balance. If we look at the oxygen atoms first, The 2 oxygen atoms in the oxygen molecule will produce 2 water molecules H2 + O2 2H2O This means that there will be 2 more hydrogen on the right hand side Left: Right: H atoms 2 H atoms 4 O atoms 2 O atoms 2 We must have used 2 molecules of hydrogen with 1 molecule of oxygen to produce 2 molecules of water. 2H2 Left: H atoms O atoms + 4 2 O2 2H2O Right: H atoms O atoms 4 2 9. Balance the following chemical equations. Note some may naturally be balanced. a. Mg + O2 MgO b. K + I2 KI c. H2+Cl2 HCl d. PbO2 + H2 Pb +H2O e. Mg + H2SO4 MgSO4 + H2 f. Li + H2O LiOH +H2 g. KOH + CO2 K2CO3 + H2O h. Cl2 + NaBr NaCl + Br2 i. AgNO3 Ag + NO2 +O2 j. C2H4 + O2 CO2 + H2O k. C3H6 + O2 CO2 + H2O l. K2CO3 + HCl KCl +H2O +CO2 m. Zn + HNO3 Zn(NO3)2 + H2 Formula Mass The formula mass of a substance (or relative formula mass) is obtained by adding the relative atomic masses of the elements in the formula. Example 1 Calculate the formula mass of calcium chloride Formula: Formula mass: CaCl2 (worked out using valencies) 40 + (35.5*2) = 111 (formula mass can be found on p4 of data book) Example 2 Calculate the formula mass of ammonium sulphate Formula: Formula mass: (NH4)2SO4 (worked out using ion group valencies) ([14+(1*4)]*2) + 32 + (16*4) Work out the formula and then calculate the formula mass for the following: a) Nitrogen, b) Methane, c) Potassium hydroxide, d) Magnesium hydroxide, e) Copper (II) sulphate f) Ammonium carbonate g) Lithium phosphate The mole The relative formula mass of a substance has no units, but in order to weigh out chemicals we need to know the units. Chemists use a special term called the mole which simply gives the units of grams to formula mass. In other words 1 mole of a substance is the same as the formula mass expressed in grams. Using the examples above 1 mole of calcium chloride is 111g and 1 mole of ammonium sulphate is 132g. For any substance the mass can be calculated from the number of moles and the formula mass. Mass of substance = number of moles x mass of one mole = number of moles x formula mass in grams Example Calculate the mass of 2.5 moles of calcium carbonate Mass in grams = number of moles X formula mass Note the number of moles (from the question) = 2.5 Formula mass for CaCO3 need to work out = 2.5 x [40+12+(16x3)] = 100g Put this information into the above equation Mass of CaCO3 in grams = 2.5x100 = 250g We can rearrange this equation to work out the number of moles or the formula mass of the substance if you have the other information If you know the mass of a substance you can work out the formula mass from the formula and data book and rearrange the equation to find the number of moles Number of moles = mass of substance in grams Formula mass Example Calculate the number of moles of water in 100g of water. Mass of water = 100g Formula mass of water = [(2XH) + O)] = [(2x1) +16)] = 18 g Number of moles = 100/18 = 5.56 Similarly if you know the mass of a substance in grams and the number of moles of this substance you can rearrange the equation to work out the formula mass of a substance. Formula mass = mass of substance in grams Number of moles of substance Example Three moles of gas X have a mass of 132g. Calculate the formula mass of gas X and suggest what it may be Mass of water = 132g Number of moles = 3 Formula mass = 132/3 = 44 the gas could possibly be CO2 Triangle of knowledge The relationship between the mass of a substance (g), the number of moles of a substance (n) and its formula mass (FM), can be summed up as follows Mass in grams m Formula mass n FM Number of moles 1. Calculate the mass of each of the following: a. 1 mole of Argon, b. 1 mole of magnesium nitride, c. 1 mole of potassium nitrate, d. 6 moles of propane e. 3.5 moles of sodium sulphate f. 0.25 moles of ammonium nitrate g. 0.016 moles of calcium iodate 2. Calculate the number of moles in each of the following a. 15g of hydrogen b. 90g of ethane c. 850g of silver (I) nitrate d. 250g of ammonium chloride e. 2.25 kg of ammonium sulphate f. 24 kg of calcium nitrate g. 1.80g of glucose. 3. Calculate the formula mass of each of the following a. 2 moles of a substance that has a mass of 36g b. 6 moles of a substance that has a mass of 180g c. 4.4 moles of a substance that has a mass of 466.4g d. 12.5 moles of a substance that has a mass of 925g e. 8.7 moles of a substance that has a mass of 2.697Kg