# In a balanced chemical equation the number that is

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In a balanced chemical equation, the number that is written in front of the element or compound, shows the mole ratio in which the reactants combine to form a product. If there are no numbers in front of the element symbol, this means the number is ’1’. e.g. N 2 + 3 H 2 2 NH 3 In this reaction, 1 mole of nitrogen reacts with 3 moles of hydrogen to produce 2 moles of ammonia. Worked Example 60: Calculating molar mass Question: Calculate the molar mass of H 2 SO 4 . Answer Step 1 : Use the periodic table to find the molar mass for each element in the molecule. Hydrogen = 1.008 g.mol 1 ; Sulfur = 32.07 g.mol 1 ; Oxygen = 16 g.mol 1 239

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13.4 CHAPTER 13. QUANTITATIVE ASPECTS OF CHEMICAL CHANGE - GRADE 11 Step 2 : Add the molar masses of each atom in the molecule M ( H 2 SO 4 ) = (2 × 1 . 008) + (32 . 07) + (4 × 16) = 98 . 09 g.mol 1 Worked Example 61: Calculating moles from mass Question: Calculate the number of moles there are in 1kg of MgCl 2 . Answer Step 1 : Write the equation for calculating the number of moles in the sample. n = m M Step 2 : Calculate the values that you will need, to substitute into the equation 1. Convert mass into grams m = 1 kg × 1000 = 1000 g 2. Calculate the molar mass of MgCl 2 . M ( MgCl 2 ) = 24 . 31 + (2 × 35 . 45) = 95 . 21 g.mol 1 Step 3 : Substitute values into the equation n = 1000 95 . 21 = 10 . 5 mol There are 10.5 moles of magnesium chloride in a 1 kg sample. Worked Example 62: Calculating the mass of reactants and products Question: Barium chloride and sulfuric acid react according to the following equa- tion to produce barium sulphate and hydrochloric acid. BaCl 2 + H 2 SO 4 BaSO 4 + 2 HCl If you have 2 g of BaCl 2 ... 1. What quantity (in g) of H 2 SO 4 will you need for the reaction so that all the barium chloride is used up? 2. What mass of HCl is produced during the reaction? Answer Step 1 : Calculate the number of moles of BaCl 2 that react. n = m M = 2 208 . 24 = 0 . 0096 mol 240
CHAPTER 13. QUANTITATIVE ASPECTS OF CHEMICAL CHANGE - GRADE 11 13.4 Step 2 : Determine how many moles of H 2 SO 4 are needed for the reaction According to the balanced equation, 1 mole of BaCl 2 will react with 1 mole of H 2 SO 4 . Therefore, if 0.0096 moles of BaCl 2 react, then there must be the same number of moles of H 2 SO 4 that react because their mole ratio is 1:1. Step 3 : Calculate the mass of H 2 SO 4 that is needed. m = n × M = 0 . 0096 × 98 . 086 = 0 . 94 g (answer to 1) Step 4 : Determine the number of moles of HCl produced. According to the balanced equation, 2 moles of HCl are produced for every 1 mole of the two reactants. Therefore the number of moles of HCl produced is (2 × 0.0096), which equals 0.0192 moles. Step 5 : Calculate the mass of HCl. m = n × M = 0 . 0192 × 35 . 73 = 0 . 69 g (answer to 2) Activity :: Group work : Understanding moles, molecules and Avogadro’s number Divide into groups of three and spend about 20 minutes answering the following questions together: 1. What are the units of the mole? Hint: Check the definition of the mole.

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