Mole Calculations. Specification points. Year 10 Moles I Quantitative Chemistry

Transcription

1 Mole Calculations Specification points Year 0 Moles I Quantitative Chemistry Relative formula mass know How to calculate the relative formula mass of a compound What a mole is and Avagadro s number How to use the relative formula mass of a substance to calculate the number of moles in a given mass of that substance and vice versa. Amounts of substances in equations- be able to: calculate the masses of substances shown in a balanced symbol equation calculate the masses of reactants and products from the balanced symbol equation and the mass of a given reactant or product balance an equation given the masses of reactants and products. Limiting reactants know Often in a reaction one of the reactants is not used up totally as it is excess That the reactant that is completely used up is called the limiting reactant because it limits the amount of products. How to explain the effect of a limiting quantity of a reactant on the amount of products it is possible to obtain in terms of amounts in moles or masses in grams. Percentage yield know That a reaction may not go to completion because it is reversible some of the product may be lost when it is separated from the reaction mixture some of the reactants may react in ways different to the expected reaction. how to calculate the theoretical amount of a product from a given amount of reactant and the balanced equation for the reaction how to calculate the percentage yield of a product from the actual yield of a reaction. Atom economy - know the atom economy is a measure of the amount of starting materials that end up as useful products and it is important when considering sustainable development/economics. how to calculate the percentage atom economy how to calculate the atom economy of a reaction from the balanced equation how to explain why a particular reaction is chosen to produce a product given appropriate data such as atom economy, yield, rate, equilibrium position and usefulness of by-products.

2 Year Moles Concentration of solutions Many chemical reactions take place in solutions. The concentration of a solution can be measured in mass per given volume of solution, eg grams per dm 3 (g/dm 3 ). Students should be able to: explain how the mass of a solute and the volume of a solution is related to the concentration of the solution calculate the mass of solute in a given volume of solution of known concentration in terms of mass per given volume of solution. Using concentrations of solutions in mol/dm 3 The concentration of a solution can be measured in mol/dm 3. The amount in moles of solute or the mass in grams of solute in a given volume of solution can be calculated from its concentration in mol/dm 3. If the volumes of two solutions that react completely are known and the concentration of one solution is known, the concentration of the other solution can be calculated. The volumes of acid and alkali solutions that react with each other can be measured by titration using a suitable indicator. Students should be able to: describe how to carry out titrations using strong acids and strong alkalis only (sulfuric, hydrochloric and nitric acids only) calculate the chemical quantities in titrations involving concentrations in mol/dm 3 and in g/dm 3 explain how the concentration of a solution in mol/dm 3 is related to the mass of the solute and the volume of the solution. Use of amount of substance in relation to volumes of gases Equal amounts in moles of gases occupy the same volume under the same conditions of temperature and pressure. The volume of one mole of any gas at room temperature and pressure (20 o C and atmosphere pressure) is 24 dm 3. The volumes of gaseous reactants and products can be calculated from the balanced equation for the reaction. Independent Study suggestions. Look at the specification points above use the textbook pages (66-89, 62-63) 2. Watch the Fuse School short 3-4 minute explanation videos on any areas you need extra help with: Balancing equations: Balancing equations II: State symbols in equations: Atom economy: Calculating % mass: Introduction to the mole: Avogadros number: Moles in equations: Using moles part I: Using moles part 2: Using moles part 3: Empirical and molecular formula: Law on conservation of mass: Calculating masses in reactions:

3 Theoretical yield and losses: % yield: Measuring loss of mass in a reaction: Molar gas volumes: Calculating gas volumes: Molar volume calculations further: Concentrations calculating: Calculating concentration 2 : Standard solutions: Titrations: Titration calculations: Dilutions: Dilutions part 2: 3. Have a go at the Balancing equation quiz 4. Now have a go at the past paper questions below. Once you have completed them you can mark the questions using the markscheme. Q.Magnesium reacts with steam to produce hydrogen gas and magnesium oxide. A teacher demonstrated the reaction to a class. The figure below shows the apparatus the teacher used. (a) (i) The hydrogen produced was collected. Describe how to test the gas to show that it is hydrogen. Test... Result... (ii) Explain why the magnesium has to be heated to start the reaction.

4 (b) The equation for the reaction is: Mg(s) + H 2 O(g) MgO(s) + H 2 (g) (i) The teacher used.00 g of magnesium. Use the equation to calculate the maximum mass of magnesium oxide produced. Give your answer to three significant figures. Relative atomic masses (A r ): O = 6; Mg = 24 Maximum mass =... g (3) (ii) The teacher s demonstration produced.50 g of magnesium oxide. Use your answer from part (b)(i) to calculate the percentage yield. If you could not answer part (b)(i), use.82 g as the maximum mass of magnesium oxide. This is not the answer to part (b)(i). Percentage yield =... % (iii) Give one reason why the percentage yield is less than 00%. () (Total 0 marks) Q2. Aspirin tablets have important medical uses.

5 A student carried out an experiment to make aspirin. The method is given below.. Weigh 2.00 g of salicylic acid. 2. Add 4 cm 3 of ethanoic anhydride (an excess). 3. Add 5 drops of concentrated sulfuric acid. 4. Warm the mixture for 5 minutes. 5. Add ice cold water to remove the excess ethanoic anhydride. 6. Cool the mixture until a precipitate of aspirin is formed. 7. Collect the precipitate and wash it with cold water. 8. The precipitate of aspirin is dried and weighed. (a) The equation for this reaction is shown below. C 7 H 6 O 3 + C 4 H 6 O 3 C 9 H 8 O 4 + CH 3 COOH salicylic acid aspirin Calculate the maximum mass of aspirin that could be made from 2.00 g of salicylic acid. The relative formula mass (M r ) of salicylic acid, C 7 H 6 O 3, is 38 The relative formula mass (M r ) of aspirin, C 9 H 8 O 4, is Maximum mass of aspirin =... g (b) The student made.0 g of aspirin from 2.00 g of salicylic acid. Calculate the percentage yield of aspirin for this experiment. (If you did not answer part (a), assume that the maximum mass of aspirin that can be made from 2.00 g of salicylic acid is 2.50 g. This is not the correct answer to part (a).) Percentage yield of aspirin =... % (c) Suggest one possible reason why this method does not give the maximum amount of aspirin ()

6 (d) Concentrated sulfuric acid is a catalyst in this reaction. Suggest how the use of a catalyst might reduce costs in the industrial production of aspirin () (Total 6 marks) Q3. Some students investigated magnesium oxide. (a) Magnesium oxide has the formula MgO. (i) Calculate the relative formula mass (M r ) of magnesium oxide. Relative atomic masses: O = 6; Mg = 24. Relative formula mass =... (ii) Calculate the percentage by mass of magnesium in magnesium oxide. Percentage by mass of magnesium in magnesium oxide =...% (iii) Calculate the mass of magnesium needed to make 25 g of magnesium oxide. Mass of magnesium =... g () (b) The students calculated that if they used 0.2 g of magnesium they should make 0.20 g of magnesium oxide. They did this experiment to find out if this was correct.

7 The students weighed 0.2 g of magnesium ribbon into a crucible. They heated the magnesium ribbon. They lifted the lid of the crucible slightly from time to time to allow air into the crucible. The students tried to avoid lifting the lid too much in case some of the magnesium oxide escaped. When all of the magnesium appeared to have reacted, the students weighed the magnesium oxide produced. The results of the experiment are shown below. Mass of magnesium used in grams Mass of magnesium oxide produced in grams (i) The mass of magnesium oxide produced was lower than the students had calculated. They thought that this was caused by experimental error. Suggest two experimental errors that the students had made. (ii) The students only did the experiment once. Give two reasons why they should have repeated the experiment. (Total 9 marks)

8 Q4. (a) A chemist was asked to identify a nitrogen compound. The chemist carried out an experiment to find the relative formula mass (M r ) of the compound. The M r of the compound was 44. Relative atomic masses: N = 4, O = 6 Draw a ring around the formula of the compound. NO NO 2 N 2 O 4 N 2 O () (b) Potassium nitrate is another nitrogen compound. It is used in fertilisers. It has the formula KNO 3. The M r of potassium nitrate is 0. Calculate the percentage of nitrogen by mass in potassium nitrate. Relative atomic mass: N = Percentage of nitrogen =... % (Total 3 marks) Q5. Toothpastes often contain fluoride ions to help protect teeth from attack by bacteria. Some toothpastes contain tin(ii) fluoride. This compound has the formula SnF 2. (a) Calculate the relative formula mass (M r ) of SnF 2. Relative atomic masses: F = 9; Sn = Relative formula mass (M r ) =...

9 (b) Calculate the percentage by mass of fluorine in SnF Percentage by mass of fluorine =... % (c) A tube of toothpaste contains.2 g of SnF 2. Calculate the mass of fluorine in this tube of toothpaste Mass of fluorine =... g () (d) The diagram represents the electron arrangement of a fluorine atom. Explain how a fluorine atom can change into a fluoride ion, F (Total 7 marks)

10 6.

11 Q7.In this question you will be assessed on using good English, organising information clearly and using specialist terms where appropriate. A student used the equipment shown to do a titration. Describe how the student should use this equipment to find the volume of sodium hydroxide solution that reacts with a known volume of acid. Include any measurements the student should make. Do not describe how to do any calculations (Total 6 marks)

12 Q8. A student carried out a titration to find the concentration of a solution of sulphuric acid cm 3 of the sulphuric acid solution was neutralised exactly by 34.0 cm 3 of a potassium hydroxide solution of concentration 2.0 mol/dm 3. The equation for the reaction is: 2KOH(aq) + H 2 SO 4 (aq) K 2 SO 4 (aq) + 2H 2 O(l) (a) Describe the experimental procedure for the titration carried out by the student (4) (b) Calculate the number of moles of potassium hydroxide used.... Number of moles =... (c) Calculate the concentration of the sulphuric acid in mol/dm Concentration =... mol/dm 3 (3) (Total 9 marks)

13 Q9.Sodium carbonate reacts with dilute hydrochloric acid: Na 2 CO 3 + 2HCl 2NaCl + H 2 O + CO 2 A student investigated the volume of carbon dioxide produced when different masses of sodium carbonate were reacted with dilute hydrochloric acid. This is the method used.. Place a known mass of sodium carbonate in a conical flask. 2. Measure 0 cm 3 of dilute hydrochloric acid using a measuring cylinder. 3. Pour the acid into the conical flask. 4. Place a bung in the flask and collect the gas until the reaction is complete. (a) The student set up the apparatus as shown in the figure below. Identify the error in the way the student set up the apparatus. Describe what would happen if the student used the apparatus shown (b) The student corrected the error. The student s results are shown in the table below. Mass of sodium carbonate in g Volume of carbon dioxide gas in cm

14 The result for 0.29 g of sodium carbonate is anomalous. Suggest what may have happened to cause this anomalous result () (c) Why does the volume of carbon dioxide collected stop increasing at 95.0 cm 3? () (d) What further work could the student do to be more certain about the minimum mass of sodium carbonate needed to produce 95.0 cm 3 of carbon dioxide? () (e) The carbon dioxide was collected at room temperature and pressure. The volume of one mole of any gas at room temperature and pressure is 24.0 dm 3. How many moles of carbon dioxide is 95.0 cm 3? Give your answer in three significant figures mol (f) Suggest one improvement that could be made to the apparatus used that would give more accurate results. Give a reason for your answer

15 (g) One student said that the results of the experiment were wrong because the first few bubbles of gas collected were air. A second student said this would make no difference to the results. Explain why the second student was correct (Total marks) Q0. Air bags are used to protect the passengers in a car during an accident. When the crash sensor detects an impact it causes a mixture of chemicals to be heated to a high temperature. Reactions take place which produce nitrogen gas. The nitrogen fills the air bag. (a) The mixture of chemicals contains sodium azide (NaN 3 ) which decomposes on heating to form sodium and nitrogen. 2NaN 3 2Na + 3N 2 A typical air bag contains 30 g of sodium azide. (i) Calculate the mass of nitrogen that would be produced when 30 g of sodium azide decomposes. Relative atomic masses (A r ): N = 4; Na = 23 Mass of nitrogen =... g (3)

16 (ii) g of nitrogen has a volume of 0.86 litres at room temperature and pressure. What volume of nitrogen would be produced from 30 g of sodium azide? (If you did not answer part (a)(i), assume that the mass of nitrogen produced from 30 g of sodium azide is 80 g. This is not the correct answer to part (a)(i).) Volume =... litres () (b) The sodium produced when the sodium azide decomposes is dangerous. The mixture of chemicals contains potassium nitrate and silicon dioxide which help to make the sodium safe. (i) Sodium reacts with potassium nitrate to make sodium oxide, potassium oxide and nitrogen. Complete the balancing of the equation for this reaction. 0Na +...KNO 3 Na 2 O + K 2 O + N 2 () (ii) The silicon dioxide reacts with the sodium oxide and potassium oxide to form silicates. Suggest why sodium oxide and potassium oxide are dangerous in contact with the skin. () (Total 6 marks)

17 ANSWERS M.(a M (i) lit splint or ignite the gas (squeaky) pop / explosion (ii) because it provides energy (for the reaction) to break bonds (in the reactants) or so the particles collide successfully ignore reference to frequency or rate of collisions because it provides the activation energy gains 2 marks (b) (i).67(g) allow correct answer (to 3 significant figures) with or without working gains 3 marks if answer incorrect allow up to 2 marks for the following steps: / 24 or moles magnesium = / 24 or 0.04(7) multiply by 40 allow ecf from incorrect ratio or incorrect number of moles 3 (ii) if correct answer from part (b)(i) used allow ecf from part (b)(i) 89.8 or 90 if.82 g used 82.4 or 82 correct answer with or without working gains 2 marks if answer incorrect, allow the following for mark:.50 /.67 (or their answer from part (b)(i)) if.82 g used:.50 /.82 2

18 (iii) any one from: ignore measurement errors not all the magnesium reacted allow the reaction may be reversible some of the magnesium oxide / product may have been left in the tube or may have been lost ignore magnesium lost different / unexpected reaction magnesium not pure [0] M2. (a) 2.6 / range 2.5 to 2.7 correct answer with or without or with wrong working gains 2 marks (accept answers between 2.5 and 2.7) if answer incorrect moles of salicylic acid = 2/38 = moles ie 2/38 or gains mark or (80/38) 2 gains mark or g 80/38 = (.304 g) gains mark (not.304g alone) 2 (b) 42. range 40.7 to 42.3 accept correct answer with or without or with wrong working for 2 marks ecf ie (. / their answer from (a)) 00 correctly calculated gains 2 marks if answer incorrect percentage yield =. / gains mark if they do not have an answer to part (a) or they choose not to use their answer then: yield = (. / 2.5) 00 () = 44 accept 44 for 2 marks with no working 2 (c) any one from: errors in weighing some (of the aspirin) lost do not allow lost as a gas not all of the reactant may have been converted to product

19 eg reaction didn t go to completion allow loss of some reactants the reaction is reversible accept other products / chemicals side reactions ignore waste products reactants impure not heated for long enough not hot enough for reaction to take place (d) any one from: use lower temperature use less fuel / energy ignore references to use of catalyst produce product faster or speed up reaction more product produced in a given time (owtte) increased productivity lowers activation energy [6] M3. (a) (i) 40 correct answer with or without working or incorrect working if the answer is incorrect then evidence of gains mark ignore units 2 (ii) 60 correct answer with or without working or incorrect working if the answer is incorrect then evidence of 24/40 or 24/(i) gains mark ecf allowed from part(i) ie 24/(i) 00 ignore units 2 (iii) 5

20 ecf allowed from parts(i) and (ii) 24/(i) 25 or (ii)/00 25 ignore units (b) (i) any two from: ignore gas is lost error in weighing magnesium / magnesium oxide allow some magnesium oxide left in crucible loss of magnesium oxide / magnesium allow they lifted the lid too much allow loss of reactants / products not all of the magnesium has reacted allow not heated enough allow not enough oxygen / air 2 (ii) any two from: ignore fair test check that the result is not anomalous to calculate a mean / average allow improve the accuracy of the mean / average improve the reliability allow make it reliable reduce the effect of errors 2 [9] M4. (a) N 2O (b) 3.8 to 4 gains full marks without working if answer incorrect 3 gains mark or 4/0 00 gains mark 2 [3] M5. (a) 57 correct answer with or without working ( ) for mark only allow (9 + 9 =) 38 for mark only ignore units 2 (b) 24.2 accept answers in the range 24 to ignore incorrect rounding after correct answer

21 (c) only without working gains mark or 38/57 00 gains mark or (9/57 00 =) 2 to 2. gains mark allow error carried forward from part(a) 38/(a) 00 gains 2 marks if calculated correctly (9/38 00 =) 3.8 gains mark accept answers in the range 0.28 to 0.3 allow error carried forward from part (b) (b)/00.2 correctly calculated ignore units 2 (d) an electron allow electrons allow electron shared / lost for mark apply list principle for additional particles is gained owtte must be linked to electron accept can hold / take in if in correct context eg it can hold another electron (in its outer shell) = 2 marks it can take an electron (from another atom) = 2 marks ignore reference to fluoride ions incorrect number of electrons gained does not gain the second mark [7] M6.

22 M7.Marks awarded for this answer will be determined by the Quality of Written Communication (QWC) as well as the standard of the scientific response. Examiners should also refer to the information in the Marking guidance. 0 marksno relevant content. Level (-2 marks)there is a simple description of using some of the equipment. Level 2 (3-4 marks)there is a description of an experimental method involving a measurement, or including addition of alkali to acid (or vice versa). Level 3 (5-6 marks)there is a description of a titration that would allow a successful result to be obtained. Examples of chemistry points made in the response could include: acid in (conical) flask volume of acid measured using pipette indicator in (conical) flask sodium hydroxide in burette

23 white tile under flask slow addition swirling colour change volume of sodium hydroxide added Extra information allow acid in the burette to be added to sodium hydroxide in the (conical) flask allow any specified indicator colour change need not be specified [6] M8. (a) any four from: sulphuric acid measure by pipette or diagram potassium hydroxide in burette or diagram if solutions reversed, award note initial reading use of indicator note final reading or amount used 4 (b) (c) = ½ or 0.5 moles H 2SO 4 react with mole KOH moles H 2SO 4 in 25.0 cm 3 = moles H 2SO 4 in dm 3 = =.36 mol/dm 3 [9] M9.(a) (delivery) tube sticks into the acid the acid would go into the water or the acid would leave the flask or go up the delivery tube ignore no gas collected

24 (b) any one from: (c) (d) bung not put in firmly / properly gas lost before bung put in leak from tube all of the acid has reacted take more readings in range 0.34 g to 0.54 g take more readings is insufficient ignore repeat (e) or accept or with no working shown for 2 marks (f) (g) use a pipette / burette to measure the acid because it is more accurate volume than a measuring cylinder or greater precision than a measuring cylinder or use a gas syringe to collect the gas so it will not dissolve in water or use a flask with a divider accept description of tube suspended inside flask so no gas escapes when bung removed they should be collected because carbon dioxide is left in flask at end and it has the same volume as the air collected / displaced M0. (a) (i) 84 / 84.5 / correct answer with or without working gains 3 marks (moles of NaN 3 =) 30/65 () moles of nitrogen = 3 () mass of nitrogen = 3 x 28 = 84 () or 2 x (23 + (3 x 4)) () 3 x (2 x4) () or []

25 2NaN 3 = 30 () 3N 2 = 84 () if answer is incorrect then look for evidence of correct working. allow ecf from previous stage mark lost for each mistake in the working if they do not have the correct answer. 3 (ii) 72 / / 72.2 allow ecf from part (i) 0.86 or ignore working 69 or 68.8 (b) (i) 2 and 5 (ii) any one from: corrosive / burns alkaline / basic do not accept acidic attacks / destroys / damages living tissue / cells allow irritant ignore reference to reactivity ignore reference to silicates ignore harmful / toxic [6]