C2 3 How Much? 3.1 – 3.4 A Recap

O 16 8 C2 3.1 Mass Numbers Mass Number Proton Number Particle Mass The Mass Number is MASSIVE! Mass Number Proton Number How many protons in oxygen? How many neutrons? How many electrons? O 8 16 Particle Mass Proton 1 Neutron Electron - 8 8 8

C2 3.2 Masses of Atoms & Moles 6 12 Ar = Relative Atomic Mass Relative to Carbon-12 Ar = usually the same as mass number, accounts for isotopes. Mr = Relative Formula Mass Add the Ar of each atom Mole = Relative Atomic Mass in grams 1 mole = Ar or Mr in grams 1 mole always contains 6.02x1023 particles NaCl 23 + 35.5 = 58.5

C2 3.3 Percentages & Formulae We can use Ar and Mr to work out the percentage of an element in a compound In MgO, how much is Mg, how much is O? Magnesium Ar = 24; Oxygen Ar = 16 Magnesium Oxide Mr = 40 (mass of Mg/total) x100 (24/40) x 100 = 60% What is the percentage mass of hydrogen in methane, CH4? x100 Mass of element in question Total mass of compound C=12, H=1 4/16=0.25 X100 = 25%

C2 3.3 Percentages & Formulae Working out the formula of a compound from its percentage composition aka the compounds Empirical Formula If 9g Aluminium reacts with 35.5g Chlorine what is the empirical formula of the compound formed? Al 13 27 e.g. Water Hydrogen Peroxide Molecular Formula H2O H2O2 Empirical Formula H2O HO 9g 27g 35.5g Simplest whole number ratio: 3xCl, 1xAl Al: = 0.33 of a mole Cl: = 1 mole AlCl3 Mass of 1 mole Mass of 1 mole

C2 3.4 Equations & Calculations We often need to know how much of each substance is involved in a chemical reaction. Firstly, eqations must be balanced: H2 + O2 → H2O Not balanced H2 + O2 → 2H2O Still not balanced 2H2 + O2 → 2H2O Balanced

C2 3.4 Equations & Calculations Now, we can calculate the reacting masses: E.g. sodium hydroxide reacts with chlorine to make bleach: If we have a solution containing 100g NaOH, how much chlorine gas will we need? 2NaOH + Cl2 → NaOCl + NaCl + H2O Sodium hydroxide Chlorine Bleach Salt Water We need 2 moles of NaOH for every 1 mole of Cl2 2NaOH + Cl2 Cl2 35.5 x2 = 71 100g NaOH 40 (1 mole) 2.5 2 26+16+1 35.5 x2 = 2.5 moles = 1.25 moles = 40 = 71 So we will need: = 81.75g Of Chlorine! 1.25 moles x 71g

Questions: How many protons would there be in: Li b) N c) Ne d) S e) Br How many neutrons do the above have? Mg + F2 → MgF2 How many moles of flourine react with 1 mole of magnesium atoms? What is the Mr of MgF2? How many times heavier is a magnesium atom than an oxygen atom? What is the percentage mass of nitrogen in the fertiliser Ammonium Nitrate (NH4NO3)? 22.55% of the mass of a sample of phosphorus chloride is phosphorous. What is the formula of phosphorous chloride? Hydrogen peroxide decomposes to form water & oxygen, write a balanced equation for this reaction. Calcium reacts with oxygen: 2Ca + O2 → 2CaO What mass of oxygen will react exactly with 60g of calcium? 1a)3; b)7; c)10; d) 16; e)35 / 2a)4; b)8; c)12; d)17; e)44 / 3a)1; b)62 / 4)1.5 times heavier / 5) 35% / 6) PCl3 / 7) H2O2 -> 2H2O + O2 / 8)24g

Answers a)3; b)7; c)10; d) 16; e)35 a)4; b)8; c)12; d)17; e)44 1.5 times heavier 35% PCl3 H2O2 → 2H2O + O2 24g