Chemical reactions doubts?

Question

The ethylene and p-xylene, obtained from oil, are material

departure for the synthesis of polyethylene terephthalate (PET) ,

which is produced by polymerization of ethylene glycol and

of dimethyl terephthalate. The ethylene, treated with oxygen in

presence of a catalyst of silver, produces ethylene oxide,

which then reacts with water in the presence of an acid to

produce ethylene glycol. Already the p-xylene is used to produce

of dimethyl terephthalate. This can be achieved by a

sequence of two reactions or by a direct reaction,

in which the oxidation is made in the presence of methanol and

a catalyst for cobalt. The reactions cited are: (see link below)

http://img182.imageshack.us/img182/8711/imagemmx2.png

Knowing that, in the production process ethylene glycol,

the yields of oxidation reactions and hydration

are 75% and 88% respectively, the mass of ethylene,

in kg, needed to produce 930Kg of ethylene glycol is

of approximately:

[Data - Masses molars: CH2 = CH2 (28g/mol);

Δ =(44gmol); HOCH2CH2OH (62g/mol)]

the answer is 636, as I come to this answer?

Answer 1

The reactions proceed:

1mol ethene ---> 1mol oxide ---> 1mol glycol

so you get THEORETICALLY 62 Kg glycol for each 28 Kg ethene.

To get 930 Kg, you would need 930 x 28/62 = 420 Kg ethene if you got 100% yield.

But your overall (ACTUAL) yield is 0.75 x 0.88 = 0.66 (66%).

So in practice you need 420/0.66 = 636 Kg ethene at the start.

Work problems like this step-by-step to avoid confusion :-))