# B calculate the concentration of the sodium hydroxide

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(b) Calculate the concentration of the sodium hydroxide solution. 5. A learner finds some sulfuric acid solution in a bottle labelled ’dilute sulfuric acid’. He wants to determine the concentration of the sulphuric acid solution. To do this, he decides to titrate the sulphuric acid against a standard potassium hydroxide (KOH) solution. (a) What is a standard solution? (b) Calculate the mass of KOH which he must use to make 300 cm 3 of a 0.2 mol.dm 3 KOH solution. (c) Calculate the pH of the 0.2 mol.dm 3 KOH solution (assume standard temperature). (d) Write a balanced chemical equation for the reaction between H 2 SO 4 and KOH. (e) During the titration he finds that 15 cm 3 of the KOH solution neutralises 20 cm 3 of the H 2 SO 4 solution. Calculate the concentration of the H 2 SO 4 solution. (IEB Paper 2, 2003) 15.2 Redox reactions A second type of reaction is the redox reaction, in which both oxidation and reduction take place. 276

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CHAPTER 15. TYPES OF REACTIONS - GRADE 11 15.2 15.2.1 Oxidation and reduction If you look back to chapter 4, you will remember that we discussed how, during a chemical reaction, an exchange of electrons takes place between the elements that are involved. Using oxidation numbers is one way of tracking what is happening to these electrons in a reaction. Refer back to section 4.11 if you can’t remember the rules that are used to give an oxidation number to an element. Below are some examples to refresh your memory before we carry on with this section! Examples: 1. CO 2 Each oxygen atom has an oxidation number of -2. This means that the charge on two oxygen atoms is -4. We know that the molecule of CO 2 is neutral, therefore the carbon atom must have an oxidation number of +4. 2. KMnO 4 Overall, this molecule has a neutral charge, meaning that the sum of the oxidation num- bers of the elements in the molecule must equal zero. Potassium (K) has an oxidation number of +1, while oxygen (O) has an oxidation number of -2. If we exclude the atom of manganese (Mn), then the sum of the oxidation numbers equals +1+(-2x4)= -7. The atom of manganese must therefore have an oxidation number of +7 in order to make the molecule neutral. By looking at how the oxidation number of an element changes during a reaction, we can easily see whether that element is being oxidised or reduced . Definition: Oxidation and reduction Oxidation is the loses of an electron by a molecule, atom or ion. Reduction is the gain of an electron by a molecule, atom or ion. Example: Mg + Cl 2 MgCl 2 As a reactant , magnesium has an oxidation number of zero, but as part of the product magnesium chloride, the element has an oxidation number of +2. Magnesium has lost two electrons and has therefore been oxidised . This can be written as a half-reaction . The half-reaction for this change is: Mg Mg 2+ + 2e As a reactant , chlorine has an oxidation number of zero, but as part of the product magnesium chloride, the element has an oxidation number of -1. Each chlorine atom has gained an electron and the element has therefore been reduced . The half-reaction for this change is:
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