Refer to table 131 you will remember that when the

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Refer to table 13.1. You will remember that when the mass, in grams, of an element is equal to its relative atomic mass, the sample contains one mole of that element. This mass is called the molar mass of that element. It is worth remembering the following: On the Periodic Table, the relative atomic mass that is shown can be interpreted in two ways. 1. The mass of a single, average atom of that element relative to the mass of an atom of carbon. 2. The mass of one mole of the element . This second use is the molar mass of the element. Table 13.2: The relationship between relative atomic mass, molar mass and the mass of one mole for a number of elements. Element Relative atomic mass (u) Molar mass (g.mol 1 ) Mass of one mole of the element (g) Magnesium 24.31 24.31 24.31 Lithium 6.94 6.94 6.94 Oxygen 16 16 16 Nitrogen 14.01 14.01 14.01 Iron 55.85 55.85 55.85 235

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13.2 CHAPTER 13. QUANTITATIVE ASPECTS OF CHEMICAL CHANGE - GRADE 11 Worked Example 55: Calculating the number of moles from mass Question: Calculate the number of moles of iron (Fe) in a 111.7 g sample. Answer Step 1 : Find the molar mass of iron If we look at the periodic table, we see that the molar mass of iron is 55.85 g.mol 1 . This means that 1 mole of iron will have a mass of 55.85 g. Step 2 : Use the molar mass and sample mass to calculate the number of moles of iron If 1 mole of iron has a mass of 55.85 g, then: the number of moles of iron in 111.7 g must be: 111 . 7 g 55 . 85 g.mol 1 = 2 mol There are 2 moles of iron in the sample. Worked Example 56: Calculating mass from moles Question: You have a sample that contains 5 moles of zinc. 1. What is the mass of the zinc in the sample? 2. How many atoms of zinc are in the sample? Answer Step 1 : Find the molar mass of zinc Molar mass of zinc is 65.38 g.mol 1 , meaning that 1 mole of zinc has a mass of 65.38 g. Step 2 : Calculate the mass of zinc, using moles and molar mass. If 1 mole of zinc has a mass of 65.38 g, then 5 moles of zinc has a mass of: 65.38 g x 5 mol = 326.9 g (answer to a) Step 3 : Use the number of moles of zinc and Avogadro’s number to calculate the number of zinc atoms in the sample. 5 × 6 . 023 × 10 23 = 30 . 115 × 10 23 Exercise: Moles and molar mass 1. Give the molar mass of each of the following elements: (a) hydrogen (b) nitrogen (c) bromine 2. Calculate the number of moles in each of the following samples: (a) 21.62 g of boron (B) 236
CHAPTER 13. QUANTITATIVE ASPECTS OF CHEMICAL CHANGE - GRADE 11 13.3 (b) 54.94 g of manganese (Mn) (c) 100.3 g of mercury (Hg) (d) 50 g of barium (Ba) (e) 40 g of lead (Pb) 13.3 An equation to calculate moles and mass in chemical reactions The calculations that have been used so far, can be made much simpler by using the following equation: n (number of moles) = m (mass of substance in g) M (molar mass of substance in g · mol 1 ) Important: Remember that when you use the equation n = m/M, the mass is always in grams (g) and molar mass is in grams per mol (g.mol 1 ).

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