4.4 alkanes and alkenes

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1.33 calculate empirical and molecular formulae from experimental data

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Calculating Molecular Formula 1 Write the element symbols S O 2  Write the masses or the % of the elements  3.2 g 6.4 g – 3.2 g = 3.2 g 3  Write the A r values of each element 32 16 4  Divide masses by At   3.2 ÷ 32 = 0.1 3.2 ÷ 16 = 0.2 5 Divide by the smallest number 0.1 ÷ 0.1 = 1 0.2 ÷ 0.1 = 2 6 Write the formula SO 2 Calculating Molecular Formula From the empirical formula, you can work out the molecular formula from the relative formula mass (M r ) of the compound. Add up the atomic masses of the atoms in the empirical formula. For example, the empirical formula of a hydrocarbon is CH 2 and its M r is 42. The mass of the atoms in the empirical formula is 14 42 ÷ 14 = 3 so you need to multiply the numbers in the empirical formula by 3 The molecular formula of the hydrocarbon is therefore C 3 H 6 .
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1.27 know that the mole (mol) is the unit for the amount of a substance

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A mole is that amount of substance that contains 6.02 x 10 23 atoms, molecules etc. This number is known as the Avogadro constant Molar masses A mole of an element will have a mass equal to its Ar but in grams A mole of a compound will have a mass equal to its Mr but in grams Moles tell you how many particles are present, and the ratio of moles tells you the ratio of particles that are represented by an equation.  You can find out how many moles are present by dividing the mass by the Mr: number of moles = mass ÷ Mr                                      
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1.35 understand how to carry out calculations involving gas volumes and the molar volume of gas

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At standard temperature and pressure, one mole of any gas will occupy 24000 cm3; also known as 24dm3. Calculating molar volume from mass Example: 976 cm 3 of oxygen was found to have a mass of 1.3 g Calculate the molar volume of oxygen, under these conditions. Volume = mass x no. of moles Mass of oxygen, O 2 , = 32.0 g. Volume of 1 g = 976 ÷ 1.3 Hence, 32 g = (976 ÷ 1.3) × 32 = 24024 cm 3 Answer: 24.0 is the volume of 32.0 g of O 2 .
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