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The mole Chemistry

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The mole Very important concept Used in almost all calculation in chemistry What is a mole? A mole is the amount of substance that contains 6x10 23 elementary units (ions, molecules and atoms). 1 mole of sodium chloride ions would contain 6x10 23 ions 1 mole of carbon dioxide would contain 6x10 23 molecules

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The mole Mole = given mass/ R.F.M R.F.M- Relative Formula Mass R.F.M= Sum of the Relative Atomic Mass (R.A.M) R.F.M of NaCl= 23+35.5= 58.5 R.F.M of ammonium sulphate- (NH 4 ) 2 SO 4 = 2x(14+4) + 32+ 4(16)= 132

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Calculating the number of moles Number of moles in 13.2g (NH 4 ) 2 SO 4 = given mass/ RFM= 13.2/132=0.1 moles

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The mole and the equation Calculate how many grams of Calcium Chloride will be formed when 2 grams of calcium is mixed with an excess of chlorine gas. To solve this problem, we must first write a balanced equation of the reaction taking place. Then determine the mole ratio. This is shown on the next slide.

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Solution Ca + Cl 2 −> CaCl 2 There is a 1:1 ratio of Calcium and Calcium Chloride This means that there will be a 1:1 mole ratio of calcium reacted to calcium chloride produced

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Solution continued No. moles calcium= given mass/ molar mass = 2g/40= 0.05 moles Since there is a 1:1 mole ration 0.05 moles of Calcium Chloride will be formed Since moles = mass/R.F.M mass= R.F.M x moles= (40+ 2(35.5) x 0.05 =111 x 0.05 = 5.55g CaCl 2 Answer= 5.55g CaCl 2

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Useful points to note 1 mole of gas, occupies 22.4 dm 3 (This will be important when calculating volumes of gases produced). Reactivity points to note Reactive metals (e.g. Group 1,2 and 3 metals and metals such as Zn, Fe and Cu) react with dilute acids to form a salt and hydrogen gas e.g. Zn (s) + H 2 SO 4(aq) −> ZnSO 4(aq) + H 2(g)

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Useful points to note More reactive metals will displace less reactive metals from solution according to order of reactivity. The order of reactivity is shown below for some of the common metals you will come across during your CSEC studies. K>Na>Ca>Mg>Al>Zn>Fe>Cu>Ag

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Useful points to note You should know most of these symbols however you may not be familiar with Zn, Fe, Cu and Ag. These are defined below: Zn-Zinc Fe- Iron Cu- Copper Ag-Silver

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Useful Points to note Fe (s) + CuSO 4 (aq ) −> FeSO 4(aq) + Cu (s) Iron will displace copper from copper sulphate.

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Problem 1 Calculate the volume of hydrogen produced when 6.5g of Zn is reacted with excess dilute sulphuric acid

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Solution Equation Zn (s) + H 2 SO 4(aq) −> ZnSO 4(aq) + H 2(g) No moles of Zn reacted = given mass/R.F.M =6.5/65 No. moles of Zn reacted = 0.1 Moles What is the mole ratio of Zn reacted to H 2 liberated

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Solution 1:1 ratio From 1:1 mole ration, the no. moles of hydrogen gas is liberated = 0.1moles Since 1 mole occupies 22.4 dm 3, then 0.1 moles should occupy 2.24 dm 3 ( 0.1 x 22.4). Answer = 2.24 dm 3

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Solution 1:1 ration From 1:1 mole ration, the no. moles of hydrogen gas is liberated = 0.1moles Since 1 mole occupies 22.4 dm 3, then 0.1 moles should occupy 2.24 dm 3 ( 0.1 x 22.4). Answer = 2.24 dm 3

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Problem 2 Calculate the volume of oxygen gas formed during the electrolysis of 18 moles of water. Equation is shown below: 2H 2 0 (l) −> O 2(g) + 2H 2(g)

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Solution Mole ratio 2 mole of water: 1 moles of oxygen gas Hence 18 moles of water would produce 9 moles of oxygen gas. We know that 1 mole of gas occupies 22.4 dm 3 Then 9 moles of oxygen will occupy 9 x 22.4 = 201.6 dm 3

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