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in Solution Most chemical reactions that occur on the earth’s surface, whether in living organisms or among inorganic substances, take place in aqueous solution. Chemical reactions carried out between substances in solution obey the requirements of stoichiometry discussed in Chapter 2, in the sense that the conservation laws embodied in balanced chemical equations are always in force. But here we must apply these requirements in a slightly different way. Instead of a conversion between masses and number of moles, using the molar mass as a conversion factor, the conversion is now between solution volumes and number of moles, with the concentration as the conversion factor. For instance, consider the reaction that is used commercially to prepare elemental bromine from its salts in solution: 2 Br � (aq) � Cl2(aq) 02 Cl � (aq) � Br2(aq) Suppose there is 50.0 mL of a 0.0600 M solution of NaBr. What volume of a 0.0500 M solution of Cl2 is needed to react completely with the Br � ? To answer this, find the number of moles of bromide ion present: 0.0500 L � (0.0600 mol L �1 ) � 3.00 � 10 �3 mol Br � Next, use the chemical conversion factor 1 mol of Cl2 per 2 mol of Br � to find moles Cl2 reacting � 3.00 � 10 �3 mol Br � a 1 mol Cl2 2 mol Br � b � 1.50 � 10 �3 mol Cl2 Finally, find the necessary volume of aqueous chlorine: 1.50 � 10 �3 mol � 3.00 � 10 �2 L solution 0.0500 mol L �1 The reaction requires 3.00 � 10 �2 L, or 30.0 mL, of the Cl2 solution.(In practice, an excess of Cl2 solution would be used to ensure more nearly complete conversion of the bromide ion to bromine. ) The chloride ion concentration after completion of the reaction might also be of interest. Because each mole of bromide ion that reacts gives 1 mol of chloride ion in the products, the number of moles of Cl � produced is 3.00 � 10 �3 mol. The final volume of the solution is 0.0800 L, so the final concentration of Cl � is [Cl � ] � 3.00 � 10 �3 mol � 0.0800 L 0.0375 M Square brackets around a chemical symbol signify the molarity of that species.
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