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Unit 2 – Chapter 4 – Quantities Review Calculating Molar Mass Molar mass = mass of 1 mol (6.02x1023 molecules/particles) of a compound or element How do you calculate the molar mass? o Write the chemical formula for the compound (if not given) o Add the average atomic mass for each element within the compound Remember to multiply by the average atomic mass for each element by any subscripts that are present ie. H2 2(1.01) = 2.02 g/mol o Unit for molar mass is g/mol Practice Problems: 1. What is the molar mass of ammonium phosphate, (NH4)3PO4? 2. What is the molar mass of calcium hydroxide, Ca(OH)2(s)? 3. What is the molar mass of chlorine, Cl2(g) 4. What is the molar mass of the polyatomic ion, hydroxide? 5. If an unknown element has a molar mass of 196.97 g/mol. What element could it be? 6. What does the term diatomic mean? a. b. Name seven elements that form diatomic molecules. How many molecules are present in one mole of any diatomic element? Converting from mass (g) to moles Take the number of grams and divide by the molar mass Eg. Calculate the number of moles in 10 g of H2(g) mass of H2 = 10 g molar mass H2 = 2.02 g/mol nH2 = ? 10 g H2 x 1 mol H2 = 4.95 mol H2 2.02 g H2 Therefore, there are 4.95 mol of H2 in 10g of H2(g) Practice Problems: 7. Convert a mass of 2.5 g of table salt (sodium chloride) to an amount in moles. 8. Convert a mass of 1.0 kg of glucose, C6H12O6(s), to an amount in moles. 9. What is the amount in moles of 25.0 g of oxygen gas? Converting from moles to mass (g) Take the number of moles and multiply by the molar mass Eg. Calculate the mass of 0.400 moles of Carbon(s) nC = 0.400 moles molar mass C = 12.01 g/mol mass of C(s) = ? 0.400 mol C x 12.01 g C = 4.80 g C 1 mol C Practice Problems: 10. Magnesium hydroxide is a base that is used in some antacids. What is the mass in grams of 0.45 mol of magnesium hydroxide? 11. Ammonia is a gas that, dissolved in water, is used as a cleaning agent. Convert 87 mmol of ammonia, NH3(g), into mass in grams. (Hint: 1 mol = 1000 mmol, you will need to convert from mmol to mol) 12. 1,4-benzenedioic acid, C8H6O4(s),is a raw material used in the manufacture of Dacron, a synthetic fibre. What is the mass in grams of 63.28 mol of 1,4benzenedioic acid? 13. If a patient is prescribed 1.0 × 10–3 mol of acetylsalicylic acid (Aspirin, C9H8O4(s)), what mass of Aspirin should she take? Calculating the number of molecules in a sample a. Use Avagadro’s Number 6.02 x 1023 molecules/mol b. Can only be used to convert from moles to molecules (1) You must convert grams to moles first! Calculating the number of atoms in a sample Find number of molecules first, using Avagadro’s Number Multiply number of molecules by total number of atoms in 1 molecule Practice Problems: 14. How many molecules are there in 3.00 mol of carbon dioxide? 15. How many atoms are there in 0.500 mol of Ar(g)? 16. How many atoms are there in 15 mol of solid carbon dioxide, in dry ice? 17. How many molecules are there in 15 g of ammonia gas, in household cleaners? Percent Composition Find molar mass of the compound Divide the molar mass of each element by the molar mass of the compound Ex. Determine the percent composition of calcium sulfate. Molar mass of CaSO4 = 40.08 + 32.07 + 4(16.00) = 136.11 g/mol Percent of Ca = mCa____ x 100% = 40.08g/mol x 100% = 41.40% mCaSO4 136.11 g/mol Percent of S = mS____ x 100% = 32.07 g/mol x 100% = 23.56% mCaSO4 136.11 g/mol Percent of O = mO____ x 100% = 64.00 g/mol x 100% = 47.02% mCaSO4 136.11 g/mol Practice Problems 18. Determine the percentage composition of sodium carbonate, Na2CO3. 19. Calculate the percentage composition by mass of sulfuric acid,H2SO4(aq), used in car batteries. 20. Calculate the percentage by mass of magnesium in magnesium hydroxide, Mg(OH)2(s), used in some antacids. 21. Iron and oxygen combine to form two different compounds. The formulas of the compounds are FeO(s) and Fe2O3(s). Calculate the percentage composition of each compound. Determining Empirical Formulas Find percent composition, if not given in question. Assume 100g of the compound. Find mass of each element present. (This is the same as the % composition) o ie. If the compound is 25% O, Then in 100 g of compound there is 25g of O. Convert mass (g) to moles for each element. Divide the number of moles for each element by the smallest number of moles o ie. If the mole ratio (Na:Cl:O) is 0.940:0.939:2.82. o Divide by 0.939 to obtain the lowest ratio 0.940 : 0.939 : 2.82 0.939 0.939 0.939 If values are within 0.05 of a whole number, round to a whole number o From above, we get 1.00: 1.00 : 3.00, since all are within 0.05 of a whole number o We get the mole ratio of Na:Cl:O = 1:1:3. If values are not close to a real number multiply all values by the same coefficient to end up with whole numbers. Ex. If the ratio was 1:2:1.33, multiply all numbers by 3 to get 1:6:4 Practice Problems 22. The percentage compositions of two antibiotics are given below. Find the empirical formula of each: a. chloromycetin: 40.87% carbon, 3.72% hydrogen, 8.67% nitrogen, 24.77% oxygen, and 21.98% chlorine. b. sulfanilamide: 41.86% carbon, 4.65% hydrogen, 16.28% nitrogen,18.60% oxygen, and 18.60% sulfur. 23. Phosphorus combines with oxygen to form two oxides. Find the empirical formula for each oxide of phosphorus if the percentage composition for each is: a. 43.6% oxygen b. 56.6% oxygen 24. Propane is a hydrocarbon that is used as fuel in barbecues and some cars. In a 26.80-g sample of propane, 4.90 g is hydrogen and the remainder is carbon. What is the empirical formula of propane? (Hint – use the given masses to determine % composition) Determining Molecular Formulas You must know the molecular mass of the final compound to determine molecular formula (you will be given this in the question) Determine the empirical formula Determine the molar mass of the empirical formula Divide the molar mass of the final compound by the molar mass of the empirical formula Multiply the subscripts of the empirical formula by the resulting number Practice Problems 25. A key ingredient of nail polish remover is found to have the following percentage composition by mass: 62.0% carbon, 10.4% hydrogen, and 27.5% oxygen. If its molar mass is 58.1 g/mol, what is its molecular formula? 26. Analysis of an air pollutant reveals that the compound is 30.4% nitrogen and 69.6% oxygen, by mass. The mass spectrograph for the pollutant shows that its molar mass is 92.0 g/mol. Determine the molecular formula and the chemical name of the polluting compound. Answers: 1. 149.12 g/mol. 2. 74.10 g 3. 70.90 g 4. 17.01 g 5. Gold (Au) 6. 2 atoms of the same element covalently bonded to form a molecule a. Cl, F, I, Br, H, O, N b. Avagadro’s number, 6.02x1023 molecules 7. 0.043 mol 8. 5.6 mol 9. 0.781 mol 10. 26 g 11. 1.5 g 12. 10.51 kg 13. 0.18 g 14. 1.81 × 1024 molecules 15. 3.01 × 1023 atoms 16. 9.0 x 1024 molecules 17. 5.3 x 1023 molecules 18. 43.38% sodium, 11.33% carbon, and 45.29% oxygen. 19. 2.1% H, 32.7% S, 65.2% O 20. 41.6% Mg 21. FeO(s) = 77.7% Fe, 22.3% O; Fe2O3(s) = 69.9% Fe, 30.1% O 22. a. C11H12N2O5Cl2 b. C6H8N2O2S 23. a. P2O3 b. P2O5 24. C3H8 25. C3H6O 26. N2O4