# chemistry a level ocr question

Europium, atomic number 63, reacts with oxygen at room temperature.

4Eu + 3O2---> 2Eu2O3

Calculate the volume of oxygen, in cm3, required to fully react with 9.12 g of europium at room
temperature and pressure.
If answer = 1080 cm3 award 2 marks

Amount of Eu = 9.12/ 152.0 = 0.06(00) mol
Amount of O2 = 0.0600 x 3/4 = 0.045(0) mol
and
Volume of O2 = 0.0450 x 24000 = 1080 cm3
Why do you need to multiply 02 by 3/4??
Original post by Orangecow
Europium, atomic number 63, reacts with oxygen at room temperature.

4Eu + 3O2---> 2Eu2O3

Calculate the volume of oxygen, in cm3, required to fully react with 9.12 g of europium at room
temperature and pressure.
If answer = 1080 cm3 award 2 marks

Amount of Eu = 9.12/ 152.0 = 0.06(00) mol
Amount of O2 = 0.0600 x 3/4 = 0.045(0) mol
and
Volume of O2 = 0.0450 x 24000 = 1080 cm3
Why do you need to multiply 02 by 3/4??

Because it's a 4:3 ratio Eu:O . So the moles of Eu are divided by 4 (to get 1 part) then multiplied by 3 (to get the required 3 parts) which is the same as multiplying by 3/4.
Original post by heptanitrocubane
Because it's a 4:3 ratio Eu:O . So the moles of Eu are divided by 4 (to get 1 part) then multiplied by 3 (to get the required 3 parts) which is the same as multiplying by 3/4.

Thankyou
How did you get 24000?
Original post by Orangecow
Europium, atomic number 63, reacts with oxygen at room temperature.

4Eu + 3O2---> 2Eu2O3

Calculate the volume of oxygen, in cm3, required to fully react with 9.12 g of europium at room
temperature and pressure.
If answer = 1080 cm3 award 2 marks

Amount of Eu = 9.12/ 152.0 = 0.06(00) mol
Amount of O2 = 0.0600 x 3/4 = 0.045(0) mol
and
Volume of O2 = 0.0450 x 24000 = 1080 cm3
Why do you need to multiply 02 by 3/4??

Ok. So in the equation the mole ratio of O2u is 3:4. So you need LESS moles of O2 than Eu for this reaction. I.e. 3/4 of a mole of O2 for every mole of Eu.
E.g. if we use a simple example
3A + 2B = 4C
This tells us that we need 3 moles of A for every 2 moles of B. So if we used 0.2 moles of B we would need 3/2 x 0.2 moles of A. So more moles if A.
In reverse if we used 0.5moles of A we would need 2/3 x0.5 moles of B. So less moles of B
Original post by unknown_matrix
How did you get 24000?

Volume= moles x 24 is used if the volume required is in decimetre cubed but since it is required in centimetres cubed, you x 24 by 1000, thus 24000, or atleast thats what my chemistry teacher said