| Date | November 2016 | Marks available | 2 | Reference code | 16N.3.sl.TZ0.2 |
| Level | SL | Paper | 3 | Time zone | TZ0 |
| Command term | State and explain | Question number | 2 | Adapted from | N/A |
Question
In a class experiment, students were asked to determine the value of x in the formula of a hydrated salt, BaCl2･xH2O. They followed these instructions:
- Measure the mass of an empty crucible and lid.
- Add approximately 2 g sample of hydrated barium chloride to the crucible and record the mass.
- Heat the crucible using a Bunsen burner for five minutes, holding the lid at an angle so gas can escape.
- After cooling, reweigh the crucible, lid and contents.
- Repeat steps 3 and 4.
Their results in three trials were as follows:
State and explain the further work students need to carry out in trial 2 before they can process the results alongside trial 1.
In trial 3, the students noticed that after heating, the crucible had turned black on the outside. Suggest what may have caused this, and how this might affect the calculated value for x in the hydrated salt.
List two assumptions made in this experiment.
Markscheme
repeat steps 3 and 4
OR
repeat step 5
OR
conduct a third heating
OR
«re»heat AND «re»weigh
water still present
OR
need two consistent readings
OR
heat to constant mass
Accept “ensure even/strong heating” for M1.
Do not accept “cleaning/washing the crucible”.
soot/carbon deposited
OR
incomplete combustion
OR
air hole of Bunsen burner closed/not fully open
Accept “using a yellow «Bunsen burner» flame” for M1.
«value of x» lower
Only award M2 if M1 correct.
all mass loss is due to water loss
all the water «of crystallization» is lost
crucible does not absorb/lose water
crystal/BaCl2 does not decompose/hydrolyse/oxidize/react with oxygen/air «when heated»
Accept “no loss of crystals/BaCl2 occurs”, “no impurities in the «weighed hydrated» salt”, “reaction goes to completion”, “heat was consistent/strong”, “crystal/BaCl2 does not absorb water during cooling”, “balance has been calibrated” or “crucible was clean at the start”.
Do not accept ”heat loss to surroundings” or “no carbon deposited on crucible”.
Reference to defects in apparatus not accepted.
Do not penalize if BaCl2.xH2O is used for BaCl2.
