A-LevelChemistryAnalytical techniques9701/31 · May/June 2021 · Q28 Marks
In Question 1 you used a titration method to determine the value of x in a hydrated salt. In Question 2 you will use a gravimetric method. In this method a sample of solid is heated to remove the water of crystallisation. You will carry out this method on a different hydrated compound, FA 4, with formula MZ•yH2O. In FA 4 the value of y is an integer. MZ•yH2O(s) → MZ(s) + yH2O(g) FA 4 is a hydrated compound, MZ•yH2O.
- (a)Method • Weigh the crucible with its lid. Record the mass. • Place between 2.40g and 2.60 g of FA 4 in the crucible and record its appearance below. • Weigh the crucible, its lid and contents and record the mass. • Without the lid, place the crucible on the pipe-clay triangle and heat gently for approximately one minute and record your observations. • Then heat more strongly for approximately four minutes. • Place the lid on the crucible and leave it to cool. You may wish to start Question 3 while you are waiting for the crucible to cool. : • Weigh the crucible, its lid and contents and record the mass. • Calculate and record the mass of FA 4, the mass of residue after heating and the mass of water lost. Keep FA 4 for use in Question 3. Results appearance of FA 4 ................................................................................................................................................... observations during heating for the first minute ........................................................................................................ .................................................................................................................................................................................... .................................................................................................................................................................................... [4][4]
- (b)(i)Calculate the number of moles of water lost when your sample of MZ•yH₂O was heated. moles of water = ................................... mol The relative formula mass of the anhydrous compound MZ is 120.4. Calculate the number of moles of MZ present in the residue. moles of MZ = ................................... mol [1][1]
- (b)(ii)Use your answers from (b)(i) to calculate the value of y in FA 4, MZ•yH2O. Show your working. y = ................................... [1][1]
- (b)(iii)State an assumption you made when calculating the value of y in the hydrated compound. [1][1]
- (c)A student suggested that the experiment would be more accurate if the crucible had been heated with the lid on for the first minute. State and explain whether you agree with the student. [1][1]
Mark scheme
Marking points from the official mark scheme (May/June 2021). The mark or mark type is shown in brackets.
- [1] Appearance FA 4: white / colourless and crystals /solid AND obs during heating: swelling / steam / (some) solid melts / dissolves / white powder forms around the edges Allow evaporation of water Allow bubbling/bubbles of water Ignore fizzing/effervescence
- [1] Unambiguous headings and units in list / table of data: • (Mass of) crucible and lid (not 'weight' in any reading) • (Mass of) crucible, lid and FA 4 (or 'contents before heating') • (Mass of) crucible, lid and residue / contents after heating • (Mass of) FA 4 (used) • (Mass of) residue • (Mass of) water (lost) Ignore mention of 1st heating if reheated Do not allow 'mass of FA 4 after heating' in third weighing. Use of lid must be consistent. Unit must be given correctly in each case.
- [1] Three weighings recorded and all listed subtractions (minimum one) correct • All weighings to the same number of dp • Mass of FA 4 used between 2.40 and 2.60 g If 4 weighings shown then all must be to the same number of dp
- [1] Accuracy mark Award if ratio between 1.80–2.10
- [1] Correctly calculates n(H2O) = mass loss in (a) / 18 and answer to 2 or more sf AND n(MZ) = mass of residue in (a) / 120.4 and answer to 2 or more sf If masses are not shown in (a) then values used must be correct
- [1] Shows use of ratio n(MZ) : n(H2O) AND y is an integer Other correct methods are available.
- [1] One of the following • All water (of crystallisation) was lost / dehydration was complete • No MZ decomposed on heating
- [1] One of the following • student incorrect and because there was no spitting (and therefore lid not required) • student correct and to catch any spitting (check observations for spitting or swelling) • student incorrect and because putting the lid on makes no difference to water (vapour) escaping
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