CAIE Chemistry (9701)

Eggshells contain a high percentage by mass of calcium carbonate, CaCO . A student wants to find out what percentage of an eggshell is calcium carbonate and uses the following method. This method uses a known excess of acid to dissolve the eggshell. The amount of unreacted acid is then determined by titration with an alkali. Assume the acid only reacts with the CaCO in the eggshell. step 1 Wash an empty eggshell with distilled water. step 2 Warm the eggshell in an oven for a few minutes until dry. step 3 Grind the eggshell into a powder. measures to three decimal places. hydrochloric acid to the conical flask. step 6 Loosely cover the conical flask and leave for two days. volumetric flask of the solution prepared in step 7 into a conical flask, add a few drops burette. The calcium carbonate in the eggshell reacts with the excess hydrochloric acid as follows. CaCO

+ H O

The excess acid reacts with the sodium hydroxide solution as follows. O

(i) Suggest how the student could confirm the eggshell is completely dry in step 2.

State why the eggshell is made into a powder in step 3 before making up the solution. Explain your answer.

Suggest why the solution is left for two days in step 6 before being used. Table 1.1. Table 1.1 titration number rough 1 2 3 16.55 32.85 16.10 32.30 0.00 16.55 0.10 16.10

Complete Table 1.1. Calculate the mean titre. mean titre = cm [2] Show your working. hydrochloric acid = mol

Calculate the amount, in mol, of CaCO that reacts with the excess of acid. Use your answer to calculate the percentage by mass of CaCO in the eggshell. Show your working. percentage by mass of CaCO = % [3] of solution in step 8.

In step 4, a conical flask is weighed using a balance accurate to three decimal places and the Calculate the percentage error in measuring the mass of this eggshell. Show your working. percentage error = %

State the effect on the percentage by mass if the eggshell is not completely dried in step 2. Explain your answer. NaOH(aq) to reduce the risk of corrosion or damage to eyes. Explain how this introduces a weakness to the experimental procedure. [Total: 14] Question 2 starts on the next page.

It is possible to measure the rate at which potassium manganate(VII), KMnO

, M = 158, diffuses through a permeable gel using the following method. petri dish containing permeable gel hole containing sample of aqueous potassium manganate(VII) step 1 A petri dish is prepared with a permeable gel. step 3 A sample of KMnO

is placed into the hole and at the same time a stopwatch is started. step 4 After 3 minutes the diameter of the coloured spot is measured and recorded. step 5 The diameter is measured every 3 minutes until there are three successive equal measurements. A student obtained the results shown in Table 2.1. Table 2.1 time diameter of the diameter increase 0 0.5 0.0 3 1.1 0.6 6 1.7 1.2 9 2.3 1.8 12 2.7 2.2 15 3.1 2.6 18 3.2 2.7 21 3.7 3.2 24 3.9 3.4 27 4.0 3.5 30 4.1 3.6 33 4.1 3.6 36 4.1 3.6

Plot a graph on the grid to show the relationship between diameter increase of the coloured spot and time. Use a cross (×) to plot each data point. Draw a line of best fit. 4.0 3.0 2.0 1.0 diameter increase of the coloured spot / cm 05 10 15 20 time / minute 25 30 35 40

(i) On the graph, circle the point which you believe to be the most anomalous.

Suggest a possible explanation for this anomaly.

Draw a suitable tangent to the line at time = 15 minutes. Calculate the gradient of your tangent. State both sets of coordinates used in your calculation. The stated coordinates must be from your tangent. Give the gradient to three significant figures. coordinates 1 coordinates 2 gradient = cm minute

Select appropriate data from Table 2.1 and calculate the average rate of diffusion of KMnO

in cm minute average rate of diffusion of KMnO = cm minute

Identify the independent variable in this experiment.

Suggest how the experiment could be made to be more reliable.

Another compound of potassium which is coloured is potassium dichromate( VI), K = 294. This compound is corrosive when aqueous. It is possible to use the method described earlier to determine the rate of diffusion of K

Predict how the graph obtained for K

would differ from that obtained for KMnO

Explain your answer.

Apart from temperature, state one variable which must be controlled when comparing the rate of diffusion of K

and KMnO

(i) Other than wearing eye protection, state one safety precaution the student should take if they were to use potassium dichromate( VI).

Another student suggests that to compare the rates of diffusion between Kand KMnO it would be easier to place solid crystals of each of these compounds into the holes in two petri dishes of permeable gel. Suggest two practical problems that this would cause. 1 2 [2] [Total: 16] Important values, constants and standards Faraday constant F = 9.65 u 10 Avogadro constant L = 6.022 u 10 electronic charge e = ?1.60 u 10?19 molar volume of gas V at room conditions ionic product of water K = 1.00 u 10?14 Group The Periodic Table of Elements hydrogen 1.0 He helium 4.0 12131415161718 34567891011123Li lithium 6.9 beryllium 9.0 atomic number atomic symbolKey name relative atomic mass sodium 23.0 magnesium 24.3 potassium 39.1 calcium 40.1 rubidium 85.5 strontiu m 87.6 caesium 132. 9 barium 137.3 francium Ra radium boron10.8 13 Al aluminium 27.0 31 Ga gallium 69.7 49 In indium 114.8 thallium 204.4 carbon 12.0 14 Si silicon 28.1 32 Ge germanium 72.6 50 Sntin 118.7 82 Pb lead 207.2 titanium 47.9 40 Zr zirconium 91.2 72 Hf hafnium178. 5 rutherfordium vanadium 50.9 41 Nb niobium 92.9 73 Ta tantalum 180.9 105 Db dubnium chromium 52.0 42 Mo molybdenum 95.9 74 W tungste n 183.8 106 Sg seaborgium manganese 54.9 43 Tc technetium rhenium 186. 2 107 Bh bohrium iron 55.8 44 Ru ruthenium 101.1 76 Os osmium 190.2 108 Hs hassium Co cobalt 58.9 45 Rh rhodium 102.9 iridium192.2 109 Mt meitnerium nicke l58.7 46 Pd palladium 106. 4 platinum 195. 1 110 Ds darmstadtium copper 63.5 47 Ag silver 107. 9 79 Au gold 197. 0 111 Rg roentgenium zinc 65.4 48 Cd cadmium 112.4 80 Hg mercury 200. 6 112 Cn copernicium flerovium livermorium nitrogen 14.0 phosphorus 31.0 33 As arsenic 74.9 51 Sb antimony 121.8 bismuth 209.0 oxygen 16.0 sulfur 32.1 34 Se selenium 79.0 52 Te tellurium 127.6 84 Po polonium fluorine 19.0 17 Cl chlorine 35.5 35 Br bromine 79.9 iodine 126.9 Ne neon 20.2 18 Ar argo n39.9 krypto n 83.8 54 Xe xenon131. 3 86 Rn radon nihonium moscovium tennessine oganesson scandium 45.0 yttriu m 88.9 57?71 lanthanoids 89?103 actinoids 57La lanthanum 138.9 89 Ac lanthanoids actinoids actinium Ce cerium 140.1 90 Th thoriu m232.0 praseodymium 140.9 91 Pa protactinium 231.0 neodymium 144.4 uranium 238.0 promethium neptunium samarium 150.4 94 Pu plutonium europium 152.0 95 Am americium gadolinium 157.3 96 Cm curium terbiu m158.9 97 Bk berkelium dysprosium 162. 5 californium holmium 164. 9 99 Es einsteinium erbium 167. 3 100 Fm fermium Tm thulium168. 9 101 Md mendelevium ytterbiu m 173. 1 102 No nobelium lutetium 175.0 lawrencium To avoid the issue of disclosure of answer-related information to candidates, all copyright acknowledgements are reproduced online in the Cambridge Assessment International Education Copyright Acknowledgements Booklet. This is produced for each series of examinations and is freely available to download at www.cambridgeinternational.org after the live examination series.