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Creado por Emily Sutton
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| Chemistry Past Paper - June 2009 | Questions on the front Model answers on the back |
| 1 The Group 2 element magnesium was first isolated by Sir Humphry Davy in 1808. (a) Magnesium has three stable isotopes, which are 24Mg, 25Mg and 26Mg. (i) Complete the table below to show the atomic structures of 24Mg and 25Mg. 24Mg - protons neutrons electrons 25Mg - protons neutrons electrons | 12, 12, 12 12, 13, 12 |
| (ii) A sample of magnesium contained 24Mg: 78.60%; 25Mg: 10.11%; 26Mg: 11.29%. Calculate the relative atomic mass of this sample of Mg. Give your answer to four significant figures. | (24x78.60 + 25x10.11 + 26x11.29)/100 Ar = 24.33 |
| (iii) Define the term relative atomic mass. | The weighted mean mass of an atom relative to 1/12th the mass of one atom of 12C. |
| (b) The reaction between magnesium and sulfuric acid is a redox reaction. Mg(s) + H2SO4(aq) MgSO4(aq) + H2 (g) (i) Use oxidation numbers to identify which element has been oxidised. Explain your answer. | Mg. Oxidation number goes from 0 to +2, increasing by 2. |
| (ii) Describe what you would see when magnesium reacts with an excess of sulfuric acid. | Mg dissolves and it bubbles/fizzes. |
| (c) Epsom salts can be used as bath salts to help relieve aches and pains. Epsom salts are crystals of hydrated magnesium sulfate, MgSO4•xH2O. A sample of Epsom salts was heated to remove the water. 1.57 g of water was removed leaving behind 1.51 g of anhydrous MgSO4. (i) Calculate the amount, in mol, of anhydrous MgSO4 formed. | mol MgSO4 = 1.51/120.4 = 0.0125 mol |
| (ii) Calculate the amount, in mol, of H2O removed. | 1.57 = 0.0872 |
| (iii) Calculate the value of x in MgSO4•xH2O. | 0.0872/0.0125 x = 7 |
| 2 This question compares the bonding, structure and properties of sodium and sodium oxide. (a) Sodium, Na, is a metallic element. Explain, with the aid of a labelled diagram, what is meant by the term metallic bonding. | Delocalised electrons between positive ions. Metallic bonding is the electrostatic attraction between the delocalised electrons and positive ions. |
| (b) Sodium reacts with oxygen to form sodium oxide, Na2O, which is an ionic compound. (i) Write the equation for the reaction of sodium with oxygen to form sodium oxide. | 4Na + O2 -> 2Na2O |
| (ii) State what is meant by the term ionic bond. | The electrostatic attraction between oppositely charged ions. |
| (iii) Draw a ‘dot-and-cross’ diagram to show the bonding in Na2O. | |
| (c) Compare and explain the electrical conductivities of sodium and sodium oxide in the solid and liquid states. | Sodium is a good conductor because it has mobile electrons |
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