Q. An ester is hydrolysed as shown by the following equation. RCOOR / + H O RCOOH + R / OH What is the percentage yield of RCOOH when 0.50 g of RCOOH (M r = 00) is obtained from.0 g of RCOOR / (M r = 50)? A % B 50% C 67% D 75% (Total mark) Q. Magnesium reacts with hydrochloric acid according to the following equation. Mg + HCl MgCl + H A student calculated the minimum volume of.56 mol dm hydrochloric acid required to react with an excess of magnesium to form 5.46 g of magnesium chloride (M r = 95.). Which of the following uses the correct standard form and the appropriate number of significant figures to give the correct result of the calculation? A 4.476 0 dm B 4.48 0 dm C 4.50 0 dm D 44.8 0 dm (Total mark) Q. Which one of the following contains the smallest number of moles of carbon dioxide gas? A B C D.65 g 0.050 m at 000 K and.0 kpa.50 dm at 7 C and 00 kpa 500 cm at 00 K and 00 kpa (Total mark) Page of 5
Q4. Which of these samples of gas contains the largest number of molecules? The gas constant R = 8. J K mol. A B C D 5.0 0 4 m at.0 0 6 Pa and 00 K 4.0 0 m at.0 0 5 Pa and 400 K.0 0 dm at.0 0 4 Pa and 500 K.0 0 dm at 4.0 0 Pa and 600 K (Total mark) Q5. The correct technique can improve the accuracy of a titration. (a) State why it is important to fill the space below the tap in the burette with solution A before beginning an accurate titration....... () (b) Suggest one reason why a 50 cm conical flask is preferred to a 50 cm beaker for a titration....... () (c) During a titration, a chemist rinsed the inside of the conical flask with deionised water. The water used for rinsing remained in the conical flask. (i) Give one reason why this rinsing can improve the accuracy of the end-point....... () (ii) Explain why the water used for rinsing has no effect on the accuracy of the titre..... () Page of 5
(d) Suggest one reason why repeating a titration makes the value of the average titre more reliable....... () (Total 5 marks) Q6. Under suitable conditions magnesium will react with dilute nitric acid according to the following equation. Mg(s) + HNO (aq) Mg(NO ) (aq) + H (g) A 0.07 g sample of magnesium was added to 6.4 cm of 0.65 mol dm nitric acid. The acid was in excess. (a) (i) Calculate the amount, in moles, of magnesium in the 0.07 g sample. () (ii) Hence calculate the amount, in moles, of nitric acid needed to react completely with this sample of magnesium. () (iii) Calculate the amount, in moles, of nitric acid originally added to this sample of magnesium. () (iv) Hence calculate the amount, in moles, of nitric acid that remains unreacted. () Page of 5
(b) In a second experiment, 0.5 mol of hydrogen gas was produced when another sample of magnesium reacted with dilute nitric acid. Calculate the volume that this gas would occupy at 98 K and 96 kpa. Include units in your final answer. (The gas constant R = 8. J K mol )............... () (c) Concentrated nitric acid reacts with magnesium to form an oxide of nitrogen which contains 0.4% by mass of nitrogen. Calculate the empirical formula of this oxide of nitrogen. Show your working................ () (Total 0 marks) Q7. A sample of pure Mg(NO ) was decomposed by heating as shown in the equation below. Mg(NO ) (s) MgO(s) + 4NO (g) + O (g) (a) A.74 0 g sample of Mg(NO ) was completely decomposed by heating. Calculate the total volume, in cm, of gas produced at 60.0 C and 00 kpa. Give your answer to the appropriate number of significant figures. The gas constant R = 8. J K mol. Total volume of gas =... cm (5) Page 4 of 5
(b) The mass of MgO obtained in this experiment is slightly less than that expected from the mass of Mg(NO ) used. Suggest one practical reason for this.......... () (Total 6 marks) Q8. (a) A sample of ethanol vapour, C H 5 OH (M r = 46.0), was maintained at a pressure of 00 kpa and at a temperature of 66K. (i) State the ideal gas equation. (ii) Use the ideal gas equation to calculate the volume, in cm, that.6 g of ethanol vapour would occupy under these conditions. (The gas constant R = 8. J K mol ) (5) (b) Magnesium nitride reacts with water to form magnesium hydroxide and ammonia. (i) Balance the equation, given below, for the reaction between magnesium nitride and water. Mg N + H O Mg(OH) + NH Page 5 of 5
(ii) Calculate the number of moles, and hence the number of molecules, of NH in 0.6 g of ammonia gas. (The Avogadro constant L = 6.0 0 mol ) (4) (c) Sodium carbonate is manufactured in a two-stage process as shown by the equations below. NaCl + NH + CO + H O NaHCO + NH 4 Cl NaHCO Na CO + H O + CO Calculate the maximum mass of sodium carbonate which could be obtained from 800 g of sodium chloride................ (4) (Total marks) Q9. Potassium carbonate can also occur as a hydrated compound, K CO.xH O. Analysis of this hydrated compound showed that it contained.5% by mass of water. Determine the value of x. Show your working................... (Total marks) Page 6 of 5
Q0. (a) Sodium hydroxide can be obtained as a monohydrate (NaOH.H O). When heated, the water of crystallisation is lost, leaving anhydrous sodium hydroxide (NaOH). A chemist weighed a clean, dry crucible. The chemist transferred.0 g of NaOH.H O to the crucible. The crucible and its contents were heated until a constant mass had been reached. The chemist recorded this mass. The experiment was repeated using different masses of the monohydrate. For each experiment, the chemist recorded the original mass of NaOH.H O and the mass of NaOH left after heating. The chemist s results are shown in the table below. Mass of NaOH.H O / g Mass of NaOH / g 0.50 0.48.0 0.79.05.4.95.06.50.8 4.0.9 4.90.4 Page 7 of 5
(i) Plot a graph of mass of NaOH.H O (y-axis) against mass of NaOH on the grid. Draw a straight line of best fit on the graph. () (ii) Use your graph to determine the mass of NaOH.H O needed to form.00 g of NaOH... g () Page 8 of 5
... NaOH +... Al +... H O... NaAl(OH) 4 + H () (iii) Use your answer from part (a) (ii) to confirm that the formula of sodium hydroxide monohydrate is NaOH.H O.......... () (b) Sodium hydroxide is used to remove grease from metal components. Sodium hydroxide cannot be used to clean components made of aluminium because it reacts with this metal. (i) Balance the equation for the reaction of aqueous sodium hydroxide with aluminium. (ii) In 986, a sealed aluminium tank exploded while being used by mistake for transporting concentrated sodium hydroxide solution. Suggest one reason why the tank exploded..... () (c) A strong alkali such as potassium hydroxide is used as the electrolyte in some alkaline batteries for household use. The electrolyte will escape if the battery casing is broken. Suggest one reason why a leak of this electrolyte is hazardous....... () (Total 9 marks) Page 9 of 5
M. D [] M. B [] M.B [] M4. B [] M5. (a) Space will fill during titration / titres or volumes added are too high Do not allow to improve accuracy without qualification. Do not allow incorrect end-point without qualification. Do not allow titres or volumes added are too low. Ignore titres or volumes added are different. (b) Less chance of losing liquid on swirling / liquid doesn t splash on swirling Do not accept easier to swirl on its own. (c) (i) Returns reagent on the sides of the flask to the reaction mixture (to ensure that all of the acid / alkali reacts) Do not allow to improve accuracy without qualification. Ignore reference to cleaning. (ii) This does not change the number of moles of reagents / water is not a reagent / water is one of the products Do not allow water does not affect the titration without qualification. Ignore water is neutral / has a ph of 7. (d) Idea that a single titration could be flawed / anomalous Do not accept will improve reliability / reproducibility / accuracy without further qualification. Allow to obtain concordant results. [5] M6. (a) (i) 0.000/.0 0 ; Penalise < sf in (a)(i); Allow.0 0.05 0. (for candidates who have used Mg as 4) Page 0 of 5
(ii) 0.0060 Allow correct answer a(i). (iii) 0.00965/ 9.65 0 ; Allow 0.009646/ 0.0096-0.0097. (iv) 0.006 moles; Allow range 0.005 to 0.007. Allow (a)(iii) (a)(ii) (must be positive). (b) PV = nrt; Allow all capitals/ lower case. V = ; M Mark is for all numbers correct. If units in answer are in dm allow this expression with 96 in denominator. 0.0 m /. dm ; M Must have correct units/ allow 00 cm. Allow min sig figs in answer. (c) O = 69.6 (%);.7 : 4.5 Use of 7/8 CE then M only. ( : ) NO Mark for formula not ratio. If NO and no working shown then allow mark. If 69.6% + NO only =. Need to see evidence of M working. Allow M conseq on the wrong M (ie max ). [0] Page of 5
M7. (a) Stage M r for Mg(NO ) = 48. Moles of Mg(NO ) = =.5 0-4 mol Extended response calculation Stage Total moles of gas produced = 5/ moles of Mg(NO ) = 5/.5 0 4 = 6.05 0 4 If ratio in stage is incorrect, maximum marks for stage is Stage PV = nrt so volume of gas V = nrt / P V = =.745 0 5 m V =.745 0 5 0 6 = 7.45 cm = 7.5 (cm ) Answer must be to significant figures (answer could be 7.4 cm dependent on intermediate values) (b) Some of the solid is lost in weighing product / solid is blown away with the gas [6] M8. (a) (i) pv = nrt () (ii) Moles ethanol = n =.6/46 (=0.096 mol) () V = nrt/p = () if V = p/nrt lose M and M4 = 8.996 0 4 (m ) () = 899 (900) cm () range = 895 905 If final answer = 0.899 award ( + M); if = 0.899 dm or if = 9 award ( + M) Note: If.6 or 46 or 46/.6 used as number of moles (n) then M and M4 not available Note: If pressure = 00 then, unless answer = 0.899 dm, deduct M and mark consequentially 5 Page of 5
(b) (i) Mg N + 6H O Mg(OH) + NH () (ii) Moles NH = (=0.055 mol) () Number of molecules of NH = 0.055 6.0 0 () [mark conseq] = 9. 0 () [range 9. 0 to 9.4 0 ] Conseq (min sig fig) 4 (c) Moles NaCl = 800/58.5 (=.68) () Moles of NaHCO =.68 () Moles of NaCO =.68/ = 6.84 () Mass of Na CO = 6.84 06 = 75 g () [range = 74 77] [450 g (range 448 454) is worth marks] Accept valid calculation method, e.g. reacting masses or calculations via the mass of sodium present. Also, candidates may deduce a direct : ratio for NaCl:Na CO 4 [] M9. Ratios 88.5 / 8. and.5 / 8 Correct answer without working scores one mark only. x = Allow K CO.H O / : ratio / one molecule of water of crystallisation. M can be awarded for a correct method using incorrect ratios. Allow correct answer if integer or decimal number. [] M0. (a) (i) Uses sensible scales Lose this mark if the plotted points do not cover half of the paper. Lose this mark if the graph plot goes off the squared paper. Plots all of the points correctly ± one small square Page of 5
Draws a best-fit line Lose this mark if the student s line includes either of the points at 0.5 / 0.48 or.5 /.8 Lose this mark if the student s line is doubled or kinked. Lose this mark if the student s line does not pass within one small square of the origin, extending the line if necessary. (ii).8 to.47 Allow answer in this range only. Answer must correspond to value from the student s graph. (iii) M Moles NaOH =.00 / 40 and Moles water = (part(b) ) / 8 Allow any correct method which uses the answer from part (b). M Ratio NaOH : H O is close to : Must compare experimental result with theoretical result to score M. (b) (i) NaOH + Al + 6H O NaAl(OH) 4 + H Ignore state symbols. (ii) Pressure build-up due to the production of hydrogen / H / gas Ignore references to the flammability / explosive nature of hydrogen. (c) (Alkali is) corrosive / caustic Allow (alkali) burns skin. Ignore harmful, dangerous. Do not allow toxic or irritant. [9] Page 4 of 5
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