#### Transcript Quantitative Chemistry

Quantitative Chemistry Foundation tier - You must be able to: • Find the relative atomic mass (mass number) of an element • Calculate the Relative formula mass of a compound • Calculate the % mass of an element of a compound Higher tier - You must be able to: • Work out the Empirical formula (HT) • Balance equations (HT) • Use chemical equations to calculate masses of reactants and products (using scale factors) (HT) • Calculate % yield (HT) • Calculate atom economy (HT) Atomic Number Atomic number (smaller number): = number of protons = number of electrons Mass Number Mass Number (larger number) = protons + neutrons (also known as Relative Atomic MASS) Relative formula mass Calculating relative formula mass To calculate this we add together the masses of all the atoms shown in the formula (N=14; H=1; Na=23; O=16; Mg=24; Ca=40) Substance Formula Formula Mass Ammonia NH3 Sodium Oxide Na2O Magnesium Hydroxide Calcium Nitrate Mg(OH)2 Ca(NO3)2 Calculating Formula Mass To calculate this we ADD TOGETHER the MASSES of ALL the ATOMS shown in the FORMULA (N=14; H=1; Na=23; O=16; Mg=24; Ca=40) Substance Formula Formula Mass Ammonia NH3 14 + (3x1)=17 Sodium Oxide Na2O (2x23) + 16 =62 Mg(OH)2 24+ 2(16+1)=58 Magnesium Hydroxide Calcium Nitrate Ca(NO3)2 40+ 2(14+(3x16))=164 Calculating % mass of an element in a compound We can use the relative atomic mass of elements and the relative formula mass of compounds to help us work out the percentage of an element in a compound. a.) Calculate % of Magnesium (Mg) in Magnesium oxide (MgO). Ar of Mg = 24; Ar of O = 16; Mr of MgO = 24 + 16 = 40 % Mg = 24/40 x 100 = 60% b.) Calculate % of Aluminium (Al) in Aluminium bromide (AlBr3) Ar of Al = 27 Ar of Br = 80 Mr of AlBr3 = 27 + (80x3) = 27 + 240 = 267 % Al = 27/267 x 100 = 10.1% Calculating Percentage Mass Calculate the percentage of Oxygen in the compounds shown below % O = (Number of atoms of O) x (Atomic Mass of Z) x 100 Formula Mass of the compound Formula Atoms of O MgO 1 K2 O 1 NaOH 1 SO2 2 Mass of O Formula Mass %age Oxygen Calculating Percentage Mass Calculate the percentage of Oxygen in the compounds shown below % O = (Number of atoms of O) x (Atomic Mass of Z) x 100 Formula Mass of the compound Formula Atoms of O Mass of O Formula Mass %age Oxygen MgO 1 16 24+16=40 16/40 x 100 = 40% K2 O 1 16 (2x39)+16= 94 16/94 x 100 = 17% NaOH 1 16 23+16+= 40 16/40 x 100 = 40% SO2 2 16 32+(2x16) = 64 32/64 x100 = 50% Empirical formula 1.) Make a column for each element 2.) Divide the mass or % by its relative atomic mass 3.) Simplify the ratio by dividing all answers by the smallest 4.) Find the simplest whole number ratio 5. Write the empirical formula Calculating Empirical formula If 9g of aluminium reacts with 35g chlorine, what is the empirical formula? (Ar of Al = 27; Ar of Cl = 35) Divide each mass by its own RAM Al: 9/27 = 0.333 Cl: 35/35 = 1 • This tells us that one 'mole' of Cl atoms combines with 1/3 mole of Al atoms. • Al needs to go up to a whole number ie 1 – • To do this we multiply the 0.333 by 3. Therefore Cl also needs to be multiplied by 3. • The ratio of Al:Cl = 1:3 • So the empirical formula is AlCl3 Calculating masses of reactants and products (Higher tier) What to do: 1. Write out a balanced equation using the formula 2. Work out relative formula mass (RFM) for each compound in the equation 3. Below the RFM write out the mass given in the qs 4. Work out the scale factor by dividing/multiplying mass given by RFM of that compound 5. Do the same thing to the value you are trying to calculate a mass for – using the scale factor Next slide shows an example Calculate masses of reactants and products using scale factors Calcium reacts with oxygen like this: 2Ca + O2 2CaO (balanced equation) Qs: What mass of oxygen will react exactly with 60g of calcium (Ar of Ca = 40; Ar of O = 16) Ca + O2 (80 + 32) 60 + ? 60/80 = 0.75 (scale factor) 32 x 0.75 = 24g of O Calculate % yield (Higher tier) The yield of a chemical reaction describes how much product is made. % yield compares the amount of product that the reaction really produces with the maximum amount that it could possibly produce. % yield = amount of product produced maximum amount of product possible x 100%. Qs: Using known masses of A and B it was calculated that a chemical reaction could produce 2.5g of product C. When the reaction is carried out only 1.5g of C is produced. What is the % yield of this reaction? % yield = amount of product max amount of product possible x 100%. % yield = 1.5 2.5 x 100 = 60% Very few chemical reactions have a 100% yield because: - the reaction may be reversible - as products form they react to form reactants again - some reactants may react to give unexpected products - some of the products may be left behind in the apparatus - the reactants may not be completely pure - some chemical reactions produce more than one product - so it maybe difficult to separate the product that we want from the reaction mixture Calculate atom economy (Higher tier) Chemical companies use chemical reactions to make products which they sell. Making as much product as possible means making less waste. The amount of the starting materials that end up as useful products is called the atom economy. The aim is to achieve maximum atom economy. % atom economy = Mr of useful product Mr of total products x 100% Ethanol (C2H5OH) can be converted into ethene (C2H4) which can be used to make polyethene. Calculate the atom economy: (Ar: C=12; H=1; O=16) C2H5OH 46 C2H4 = 28 + H2O = 18 % atom economy = 28/46 x 100 = 61% It is important to maximise atom economy to conserve resources and reduce pollution Balancing equations (Higher tier) a.) Ca +O b.) N2 + CaO H2 NH3 c.) NaOH + Cl2 NaOCl + NaCl + H2O