GCSE Qs - Reacting Amount Calculations

14

Please can you help me? I have tried to work this out, but keep getting wrong answers, which are different from the ones at the back of the CPG workbook.

6. Answer the following question about the equation 2Al2O3 --> 4Al+302
How much aluminium oxide would be needed to make the following amounts of aluminium?

a) 1kg b) 2kg c) 4.5 kg d) 1 tonne (1000 kg)

Thanks!

Reply 1

moojoo

8

molar ratio of aluminium oxide to aluminium is 1:2,

mol of aluminium for 1kg = mass/molar mass = 1000g/27 g mol^-1 = 37.04 mol

therefore mol of aluminium oxide = 0.5 x 37.04 = 18.52 mol

hence mass = mol x molar mass of aluminium oxide
= 18.52 x 102 = 1889.04g = 1.89 kg (3SF)

for the other parts, simply work out what fraction they are of 1kg and multiply appropriately to get those answers. hope that helped!

Reply 2

mackin boi

1

All in kilograms yea...
From equation ratio of Al to Al2O3 is 4:2 = 2:1 (therefore multiply moles of Al my .5)
a) (1/27) = number of moles of Aluminium
Moles of Al2O3 = 1/54
Mass of Al2O3 = (1/54)*RMM of Al2O3 = (1/54)*102 = 17/9 Kg
If you do the same thing for all the masses then it should be fine.... is that answa right?

Reply 3

moojoo

8

mackin boi

All in kilograms yea...
From equation ratio of Al to Al2O3 is 4:2 = 2:1 (therefore multiply moles of Al my .5)
a) (1/27) = number of moles of Aluminium
Moles of Al2O3 = 1/54
Mass of Al2O3 = (1/54)*RMM of Al2O3 = (1/54)*102 = 17/9 Kg
If you do the same thing for all the masses then it should be fine.... is that answa right?

yeah that works, but surely to be correct you must convert from kg to g, because the molar masses are in g mol^-1, even though the factor of 1000 cancels out at the end?

Reply 4

NickiM

OP

14

Yep, thanks! You're right :smile:

Reply 5

mackin boi

1

moojoo

yeah that works, but surely to be correct you must convert from kg to g, because the molar masses are in g mol^-1, even though the factor of 1000 cancels out at the end?

i was always taught to just forget the factor because it takes up time in the xam....and it sort of complicates it ... s'all gud

Reply 6

moojoo

8

kool, yeah that's fair enough i suppose it does take time

GCSE Qs - Reacting Amount Calculations

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