Using the molar volume in calculations with chemical equations
2 marks · EasyExplain why the volume of gas collected must be converted to dm³ before using the molar volume of 24 dm³ in calculations.
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Must show: Reference that molar volume is in dm³, Matching units needed for formula
Also accept: You must convert to dm³ so units match the molar volume.; If you use cm³, you need to use 24,000 cm³ not 24 dm³.
Common mistakes: Just says to convert without linking to units or calculation; Thinks any unit can be used
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Marking points
B1 · 1 mark · States that the molar volume is given in dm³.
B2 · 1 mark · Explains that the units in the formula must match, or calculation would be wrong if units don't match.
Worked solution
The molar volume of 24 is in dm³, so the volume of gas must also be in dm³ for the units to match and for the calculation to be correct. If the gas volume is not in dm³, the number of moles calculated will be wrong.