Chemical calculations Keywords atomic number mass number isotope

Chemical calculations Keywords: atomic number, mass number, isotope, formula mass, percentage composition, concentration Do Now: How many protons, neutrons and electrons are there in Atomic and formula mass Isotopes are elements with the same atomic number (no. of protons) and different mass numbers (no of protons + neutrons). This means they have different numbers of neutrons. Formula mass is the sum of the mass of all atoms in a formula. Example: Na 2 O Number of Na atoms: 2 Mass of Na atoms: 2 × 23 = 46 Number of O atoms: 1 Mass of O atoms: 1 × 16 = 16 Total formula mass = 46 + 16 = 62

Relative atomic mass Atomic mass in the periodic table is an average of all isotopes that naturally exist of an element. You can calculate atomic mass of samples Eg. Calculate the atomic mass of a sample containing 75% (75 x 35) + (25 x 37) 100 Calculate the atomic masses of the following samples give answers to 1 decimal place : 1. 50% 2 H and 50% 1 H 2. 25% 21 Na and 75% 23 Na 3. 40% 6 Li and 60% 7 Li 4. 20% 14 O, 60% 16 O and 20% 18 O 5. 31. 5% 18 F and 58. 5% 20 F 35 Cl and 25% 37 Cl

Percentage composition can be calculated by: % composition = (mass of element ÷ formula mass) x 100 Example: Cu in Cu. O Step 1 = mass of element Mass of Cu = 63. 5 Step 2 = formula mass Mass of Cu. O = 63. 5 + 16 = 79. 5 Step 3 = substitute in! % composition = (63. 5 ÷ 79. 5) × 100 = 79. 9 % Calculate the % composition of the named elements in the following compounds 1. N in NH 3 2. C in CH 4 3. O in Na 2 O 4. Li in Li. OH 5. Na in Na 2 CO 3 Moles Mole (mol) = a unit for the amount of substance. The number of molecules within 1 mol of any substance is equal to the Avogadro constant Equation: Moles = mass of substance (g) ÷ formula mass Calculate the moles in each of the following: 1. 24 g of Mg 2. 112 g of CO 2 3. 117 g Na. Cl 4. 45 g of Al(OH) 3 5. 1 kg of Fe(NO 3)2

Concentration Calculate the concentration in g/dm 3 for: a) 50 g of sodium chloride in 2. 5 dm 3 of water b) 9 g of potassium chloride in 5. 6 dm 3 of water c) 40 g of lithium chloride in 0. 02 dm 3 of water d) 1. 8 g of sodium carbonate in 0. 82 dm 3 of water e) 5. 8 g of sodium sulfate in 400 cm 3 of water Balancing symbol equations A subscript in a chemical formula means there are x number of the atom before within the molecule. THESE CANNOT BE CHANGED A multiplier in front of a formula means x lots of that molecule THESE CAN BE CHANGED Balance each of the following symbol equations. 1. H 2 + Cl 2 HCl 2. H 2 + Br 2 HBr 3. Na + Cl 2 Na. Cl 4. Li + F 2 Li. F 5. K + Cu. SO 4 K 2 SO 4 + Cu