Student Book links Specification links Links to prior learning Suggested teaching order

Teaching plan 5.1.2 Molecular formulae Student Book links Specification links Links to prior learning Suggested teaching order 5.1.2 5.03 5.04 5.05 Core practical 1 Learning objectives Empirical formulae Molecular formulae Relative molecular mass (M r ) 1. The ideal gas equation 2. Measuring the molar volume of a gas Students should be able to: identify what is meant by the term molecular formula and be able to calculate molecular formulae from experimental data identify that the molar mass of a substance is the mass per mole of the substance in g mol 1 calculate the molar mass of a gas or volatile liquid in SI units, using the ideal gas equation pv = nrt Key terms Molecular formula Molar mass Maths links Practical skills Measure the molar volume of a gas (CPAC 1) Suggested activity Practical 3: Measure the molar volume of a gas (Core practical 1; Practical techniques 1, 4, 11, CPAC 1a, 2a b, 3a b, 4a b) Digital learning ideas Use an appropriate number of significant figures (B.1.1). Homework, practice and support: Maths Handling data Precision Homework, practice and support: Maths Handling data Significant figures Pre-unit homework suggestions Read Section 5.1.2 in the Student Book. Suggested starter activities Equipment Teacher notes 1. Students write down the ideal gas equation and identify what each term means, including the relevant units. Student Book Check the accuracy of students notes.. 1

Teaching plan 5.1.2 Molecular formulae 2. Students read about measuring the molar volume of a gas and draw appropriate tables for the practical activity results. 3. Students carry out practice calculations in which they have to convert between the amounts of substances in moles and mass. They then self- or peer-assess their answers against the mark scheme. This activity refers to a suggested main activity. Take feedback on success. Suggested main activities Equipment Teacher notes 1. Students carry out a practical investigation to measure the molar volume of a gas. Students measure the mass of a syringe containing a small volume (< 0.2 cm 3 ) of propanone and then introduce it to a gas syringe in a syringe oven at approximately 60 C. They measure the volume of gas produced and the new mass of the syringe. Students calculate the amount of propanone used from its M r and the mass introduced and then calculate the molar volume from the gas syringe reading. 2. Students carry out a practical investigation to measure the molar volume of a gas. Students measure the mass of the butane cylinder and then introduce some gas to the gas syringe. They measure the volume of gas and the new mass of the cylinder. Syringe oven, thermometer, gas syringe with rubber septum, syringe with needle, mass balance (2 d.p.), propanone, eye protection Butane camping gas cylinder with valve and tubing, gas syringe, mass balance (2 d.p.), thermometer, eye protection Instruct students to wear eye protection. Instruct students to wear eye protection. Students calculate the amount of butane used from its M r and the mass introduced and then calculate the molar volume from the gas syringe reading. 3. Students analyse the results of their experiment. Student Book Molar volume at STP = 22.41 dm 3 (at 20 C and mean atmospheric pressure at sea level, V m = 24.04 dm 3 ).. 2

Teaching plan 5.1.2 Molecular formulae Suggested plenary activities Equipment Teacher notes 1. Students identify sources of error in their experiments and suggest ways to reduce these errors. 2. Students calculate molar volumes using supplied data from experiments. They then self- or peer-assess their answers against the mark scheme. Each group of students could look at their own results, or they could discuss their results with another group. Likely sources of error include errors in determining mass and volume. Students could be asked to complete a partially completed table. 3. Students answer the questions from Section 5.1.2 in the Student Book. Homework suggestions Student Book Answer the questions from Section 5.1.2 in the Student Book. Write an evaluation of your experiment to determine molar volume. Read Section 5.2.1 in the Student Book, ready for the starter activity in the next lesson. Wider reading Read about the gas laws, including Boyle s law, Charles s law and Gay-Lussac s law. Support ideas Structure the calculations needed to determine V m. Extension ideas You may prefer to set these questions for homework. Research the volume of hydrogen used to fill the Hindenburg airship and use the experimental value for V m to calculate the mass of hydrogen used. Potential misconceptions Some students may recall a value of V m from their GCSE course and may not appreciate that it varies according to temperature and pressure. Links to future learning The mole and the Avogadro constant (Specification links 5.1 5.2) Differentiation for AS students This entire topic is relevant to both AS and A level students.. 3

Teaching plan 5.1.2 Molecular formulae Notes. 4

Practical 3 Student sheet Measure the molar volume of a gas Practical 3: Measure the molar volume of a gas Objective To find the volume of one mole of carbon dioxide gas Safety Wear eye protection. Ensure the delivery tube does not become blocked. Ethanoic acid will sting if it gets into cuts in the skin. All the maths you need Recognise and make use of appropriate units in calculations. Use ratios, fractions and percentages. Translate information between graphical, numerical and algebraic forms. Plot two variables from experimental or other data. Equipment boiling tube stand and clamp bung fitted with delivery tube to fit boiling tube water bath for gas collection 100 cm 3 measuring cylinder 50 cm 3 measuring cylinder test tube mass balance (2 d.p.) 1 mol dm 3 ethanoic acid Powdered calcium carbonate Diagram All users will need to review the risk assessment information and may need to adapt it to local circumstances. 1

Practical 3 Student sheet Measure the molar volume of a gas Procedure 1. Place 30 cm 3 of 1 mol dm 3 ethanoic acid in the boiling tube. 2. Set the apparatus up as shown in the diagram. 3. Place approximately 0.05 g of calcium carbonate in a test tube. 4. Weigh the test tube and its contents accurately. 5. Remove the bung from the boiling tube and tip the calcium carbonate into the boiling tube. Quickly replace the bung in the boiling tube. 6. Once the reaction is over, measure the volume of gas collected in the measuring cylinder. 7. Reweigh the test tube that had contained the calcium carbonate. 8. Repeat the experiment six more times, increasing the mass of calcium carbonate by about 0.05 g each time. Do not exceed 0.40 g of calcium carbonate. Analysis of results 1. Record your results in a suitable way. 2. Plot a graph of mass of calcium carbonate (x) against volume of carbon dioxide collected (y). Draw a straight line of best fit this line must pass through the origin. 3. Use the graph to find the volume of carbon dioxide that would be made from 0.25 g of calcium carbonate. 4. In this reaction, one mole of calcium carbonate makes one mole of carbon dioxide. Calculate the number of moles of calcium carbonate in 0.25 g and hence calculate the volume of one mole of carbon dioxide gas in dm 3. Learning tips Ensure that points plotted on a graph take up more than half the available space on both scales. Axes must occupy half of the space on graph paper. Keep scales simple: one big square as 5 or 10 or 20 is ideal, one big square as 3 or 7 is very difficult to plot on and often leads to errors. Always consider whether or not the graph line should go through the origin. Straight lines should be drawn with aid of a rule one long enough to cover the full length of the line. Questions 1. Write a chemical equation for the reaction between ethanoic acid, CH 3 COOH and calcium carbonate. 2. Why is it more accurate to find the mass of the calcium carbonate used by weighing the test tube with calcium carbonate in, then tipping it out and reweighing the test tube, rather than weighing the empty tube at the start? 3. Identify the major source of error caused by the procedure used. 4. What change to the procedure/apparatus could be made to eradicate this error? 5. Carry out two calculations to show that the ethanoic acid was in excess in all experimental runs. Exam-style questions 1. A student repeated the experiment you have carried out using the apparatus shown in the diagram. (a) The balance used by the student was accurate to 0.005 g. Calculate the percentage error in the mass of calcium carbonate when the recorded mass of calcium carbonate was 0.30 g. (b) Carbon dioxide gas is slightly soluble in water. What effect would this have on the calculated molar volume of carbon dioxide? Explain your answer. State a change to the apparatus used that will prevent this problem. (2) (3) All users will need to review the risk assessment information and may need to adapt it to local circumstances. 2

Practical 3 Student sheet Measure the molar volume of a gas (c) Another student suggested that the first few bubbles of gas should not be collected. Explain why they are incorrect to state this. (3) All users will need to review the risk assessment information and may need to adapt it to local circumstances. 3

Practical 3 Teacher sheet Measure the molar volume of a gas Practical 3: Measure the molar volume of a gas Objective To find the volume of one mole of carbon dioxide gas Safety Wear eye protection. Ensure the delivery tube does not become blocked. Ethanoic acid will sting if it gets into cuts in the skin. Procedure 1. Place 30 cm 3 of 1 mol dm 3 ethanoic acid in the boiling tube. 2. Set the apparatus up as shown in the diagram. 3. Place approximately 0.05 g of calcium carbonate in a test tube. 4. Weigh the test tube and its contents accurately. 5. Remove the bung from the boiling tube and tip the calcium carbonate into the boiling tube. Quickly replace the bung in the boiling tube. 6. Once the reaction is over, measure the volume of gas collected in the measuring cylinder. 7. Reweigh the test tube that had contained the calcium carbonate. 8. Repeat the experiment six more times, increasing the mass of calcium carbonate by about 0.05 g each time. Do not exceed 0.40 g of calcium carbonate. Answers to questions Specification links Core practical 1 Practical techniques 1, 4, 11 CPAC statements 1a, 2a, 2b, 3a, 3b, 4a Notes on procedure Use of over 0.40 g of calcium carbonate will result in the volume of gas produced exceeding the capacity of the measuring cylinder. If larger masses of calcium carbonate are to be used (such as if only a 1 d.p. mass balance is available) then a larger measuring cylinder will be required. Ensure the bung is replaced quickly during step 5 to minimise gas loss. Use of a stronger acid will result in a faster reaction and so a greater loss of gas when the calcium carbonate is added to the acid. 1. CaCO 3 + 2CH 3 COOH Ca(CH 3 COO) 2 + CO 2 + H 2 O 2. Allows for the mass of any calcium carbonate that remains in the test tube after tipping it out. 3. Gas loss before replacing the bung. 4. Use tube containing the acid inside the vessel containing the calcium carbonate tip to mix the reagent. 5. When 0.40 g of calcium carbonate is used: moles CaCO 3 = 0.4 / 100.1 = 0.003996 moles ethanoic acid = c v = 1 30/1000 = 0.03 moles acid > 2 moles calcium carbonate hence ethanoic acid in excess. All users will need to review the risk assessment information and may need to adapt it to local circumstances. 1

Practical 3 Teacher sheet Measure the molar volume of a gas Answers to exam-style questions 1. (a) calcium carbonate: ((0.005 2) / 0.3 ) 100 = 3.33% (b) Volume of gas collected lower. Hence molar volume of gas lower. This can be prevented by collecting the gas in gas syringe. (c) The first bubbles of gas collected are air. These have been displaced from the delivery tube by the carbon dioxide made. The volume of carbon dioxide remaining in the delivery tube at the end is the same as the volume of air displaced. Sample data Mass calcium carbonate/g Volume of carbon dioxide collected/cm 3 0.05 11 0.11 27 0.17 32 0.21 50 0.24 59 0.32 74 0.33 80 All users will need to review the risk assessment information and may need to adapt it to local circumstances. 2

Practical 3 Technician sheet Measure the molar volume of a gas Practical 3: Measure the molar volume of a gas Objective To find the volume of one mole of carbon dioxide gas Safety If new delivery tubes need to be made, take great care when pushing glass tubing through the rubber bung. A suitable lubricant should be used. Equipment per student/group boiling tube Notes on equipment stand and clamp bung to fit boiling tube, fitted with delivery tube water bath for gas collection 100 cm 3 measuring cylinder 50 cm 3 measuring cylinder test tube mass balance (2 d.p.) Place a test tube rack or 250 ml beaker on the balance to support the test tube while being weighed. 1 mol dm 3 ethanoic acid The students will require about 250 cm 3 each. powdered calcium carbonate Notes The students will require about 2 g each. All users will need to review the risk assessment information and may need to adapt it to local circumstances. 1

Assessment 5 Formulae, equations and amounts of substance 1 Which of the following contains the most atoms? A 27.0 g of H 2 O B 4.4 g of H 2 C 32.0 g of O 2 D 34.0 g of H 2 O 2 Your answer (Total for Question 1 = 1 mark) 2 Chlorine gas can be prepared in the laboratory by adding an excess of concentrated hydrochloric acid to potassium manganate(vii) crystals: 8HCl + 2KMnO 4 3Cl 2 (g) + 2MnO 2 + 4H 2 O + 2KCl In one such preparation, 1.90 g of potassium manganate(vii) was used. (a) Calculate the molar mass of potassium manganate(vii). (b) Calculate the amount (in moles) of potassium manganate(vii) used. (c) Calculate the amount (in moles) of chlorine produced. (d) Calculate the volume of chlorine produced. (The molar volume of any gas at room temperature and pressure = 24 000 cm 3 mol 1 ) 1

Assessment 5 Formulae, equations and amounts of substance (e) Calculate the maximum mass of manganese(ii) oxide, MnO 2, that would be produced. (Total for Question 2 = 5 marks) 3 Copper(II) chloride solution, CuCl 2 (aq), can be prepared by reacting excess powdered copper(ii) oxide, CuO(s), with hot hydrochloric acid: CuO(s) + 2HCl(aq) CuCl 2 (aq) + H 2 O(l) Unreacted copper(ii) is then removed by filtration, and water is evaporated from the filtrate to leave crystals of copper(ii) chloride-2-water, CuCl 2 2H 2 O. In one such preparation, 50 cm 3 of 0.50 mol dm 3 hydrochloric acid is used. (a) Calculate the amount (in moles) of hydrochloric acid used. (b) Calculate the minimum amount (in moles) of copper(ii) oxide needed. (c) Calculate the mass of copper(ii) oxide needed, if an excess of 20% is necessary. (2) (d) Calculate the molar mass of copper(ii) chloride-2-water, CuCl 2 2H 2 O. (e) 1.81 g of copper(ii) chloride-2-water is obtained. Calculate the percentage yield. (2) (Total for Question 3 = 7 marks) 2

Assessment 5 Formulae, equations and amounts of substance 4 An oxide of nitrogen contains 30.4% nitrogen by mass. (a) State the meaning of the term empirical formula. (b) Determine the empirical formula of this oxide of nitrogen. (3) (c) The relative formula mass of this oxide of nitrogen is 92.0. Determine its molecular formula. (Total for Question 4 = 5 marks) 5 A white precipitate forms when aqueous sodium sulfate solution is mixed with aqueous barium sulfate solution. Which of the following is the ionic equation for the reaction? A B C D Ba 2+ (aq) + SO 2 4 (aq) BaSO 4 (s) Ba + (aq) + SO 4 (aq) BaSO 4 (s) Na 2+ (aq) + 2Cl (aq) NaCl 2 (s) Na + (aq) + Cl (aq) NaCl(s) Your answer (Total for Question 5 = 1 mark) 3

Assessment 5 Formulae, equations and amounts of substance 6 Phenolphthalein and methyl orange are two indicators used in acid-base titrations. Which of the following shows their correct colours in alkaline and acidic solutions? Phenolphthalein Methyl orange Alkaline Acidic Alkaline Acidic A colourless pink red yellow B pink colourless red yellow C pink colourless yellow red D colourless pink yellow red Your answer (Total for Question 6 = 1 mark) TOTAL FOR ASSESSMENT = 20 MARKS 4

Mark scheme 5 Formulae, equations and amounts of substance Question number Answer Additional guidance Mark 1 A (Total for Question 1 = 1 mark) Question number Answer Additional guidance Mark 2(a) 39.1 + 54.9 + (2 16.0) = 158.0 g mol 1 Answer should be written to one decimal place. 2(b) 1.90 158.0 = 0.0120 mol 1.20 10 2 mol 2(c) 0.0120 2 3 = 0.0180 mol Allow 1.5 answer 2(b) 2(d) 0.0180 24000 = 432 cm 3 Allow 24000 answer 2(c) 2(e) 86.9 0.0120 = 1.04 g Allow 86.9 answer 2(b) (Total for Question 2 = 5 marks) 1

Mark scheme 5 Formulae, equations and amounts of substance Question number Answer Additional guidance Mark 3(a) 0.50 50 1000 = 0.025 mol 2.5 10 2 mol 3(b) 0.025 2 = 0.0125 mol 1.25 10 2 mol 3(c) excess = 0.0125 1.2 = 0.015 (2) molar mass of CuO = 63.5 + 16.0 = 79.5 mass = 0.015 79.5 = 1.19(25) g 3(d) 63.5 + (2 35.5) + (2 18.0) = 170.5 3(e) expected yield = 0.0125 170.5 = 2.13 % yield = 1.82 100 2.13 = 85.0% 2.13125 84.9% if 2.13125 used (2) (Total for Question 3 = 7 marks) Question number Answer Additional guidance Mark 4(a) Simplest whole-number ratio of the atoms of each element in a compound 4(b) O = 100 30.4 = 69.6% N: 30.4 14.0 = 2.17 O: 69.6 16.0 = 4.35 N: 1 O: 2 NO 2 4(c) factor = 92.0 46.0 = 2 molecular formula is N 2 O 4 (3) (Total for Question 4 = 5 marks) 2

Mark scheme 5 Formulae, equations and amounts of substance Question number Answer Additional guidance Mark 5 A (Total for Question 6 = 1 mark) Question number Answer Additional guidance Mark 6 C (Total for Question 6 = 1 mark) 3