Draw a sealed delivery setup, convert carbonate mass to moles, read the corrected gas volume and divide volume by amount with consistent conditions and units.
Magnesium carbonate powder reacts with hydrochloric acid.
The value of molar volume calculated in (b)(iv) was lower than the student expected. Give two reasons for the value being lower than expected. Assume that the correct amounts of hydrochloric acid and magnesium carbonate were used. (2)
An answer that makes reference to any two of the following points:
- loss of gas before the bung is inserted / other named reason
(1)
- some carbon dioxide dissolved in the water
(1)
- temperature of the lab was lower than standard temperature.
(1)
Do not allow "loss of gas" unless a reason is given eg delivery tube not positioned correctly so not all goes into measuring cylinder, badly fitting bung Ignore leaks
Allow gas for carbon dioxide
Ignore higher pressure
Do not award higher temperature / lower pressure / suck-back
Ignore impurities in MgCO3
Ignore incomplete reaction
Comment: Apply the list principle ie
- 1 correct answer and 1 do not award answer scores 1
- 2 correct answers and 1 do not award answer scores 1
- 2 correct answers and 2 do not award answers scores 0
- 2 correct answers and 1 ignore scores 2
(2)