URGENT: Help!!! (1 Viewer)

[7926bunting]

Hello,

Can someone please check my calculations and make corrections where needed?

Q:
Calculate the mass of ethanol that can be produced by the fermentation of 2.5g glucose? If 150mL of water is used in this reaction, what is the ethanol content of the product mixture?

A:
C6H12O6 (aq) --> 2C2H5OH (aq) + 2CO2 (g)
ethanol = 2 x mass glucose/molar mass of glucose
= 2 x 2.5/72.06+12.096+96
= 2 x 2.5/180.156
= 5/180.156
= 0.02775
= 0.02775... x molar mass of ethanol
= 0.02775... x 46.068
= 1.28 g

...so is working out the ethanol content of the product mixture simply:

1.28/150 x 100 = 0.85%

Can anyone let me know if this is right...or if I have done something wrong, can you please make corrections?

Thanks

 

[Pwnage101]

Your final answers seem correct, just a couple of notes:

- Your notation is a little dodgy, 'ethanol' should be 'moles of ethanol', and there shouldn't be equality at your 6th equal sign, since from here on you are calculating 'mass of ethanol'.

- To work out the ethanol content, i'm not sure what the solution would show, but i would use 1.28/(1.28+150) ---------> Since we want overll per centage, the denominator, in my opinion, should include the 1.28. In any case, to 3 significant figures, both produce the same answer, however strict markers may not give full marks.

- I would include the assumption: "assume density of water is 1 g/mL"

- Note: You incorrectly wrote "1.28/150 x 100 = 0.85%". It is clear that 1.28/150 x 100 = 0.85 (no % sign). What you should have written is: "1.28/150 = 0.85% (=0.0085)". i.e. There is no need to multiply by 100 since "%" i.e. "per cent" means exactly that - 1 per 100, so if you wanted to show your conversion factor, note that 100% = 1, so multiplying by 100% does not change the number, so you could have writtten "1.28/150 x 100% = 0.85%" - This also explains where you ot the % sign, and that you didn't pluck it out of thin air.

Markers are looking for good coherent solutions, not just correct answers.

 

[7926bunting]

Thanks for your suggestions.

What bothers me about this question is that shouldn't the concentration of ethanol be at 14-15% given that yeast cannot survive once ethanol reaches this concentration?

 

[Pwnage101]

This is a calculation question, most likely made up by the author and not from experimental evidence, so no, does not need to be 14-15% for this type of question. That said, 15% is a maximum, there is no minimum.

 

[7926bunting]

But if the question were to also state:

A student carries out a fermentation reaction. The aim of the experiment is to determine whether or not yeast can produce ethanol solutions greater than 15% ethanol. Calculate the mass of ethanol that can be produced by the fermentation of 2.5g glucose? If 150mL of water is used in this reaction, what is the ethanol content of the product mixture? (dry yeast is also used) Assess the validity of the experimental setup?

I'm relatively sure my calculations are accurate, but as the question states the aim is to determine whether or not yeast can produce ethanol solutions greater than 15% ethanol and taking into account that yeast cannot survive once ethanol reaches 15% concentration, I feel as though something must be wrong.