Biology maths question

Hi, can anyone please help me with this question?

A 2g sample of biomass was fully combusted in a calorimeter. The volume of water in the calorimeter was 100 cm*3. The increase in temperature recorded was 15.7 °C
4.18 J of energy are needed to increase the temperature of 1cm*3 of water by 1°C
Use this information to calculate the heat energy released in KJ per g of biomass. Show your working. (3 marks)

Reply 1

bl0bf1sh

15

Original post by Laychips

Hi, can anyone please help me with this question?

A 2g sample of biomass was fully combusted in a calorimeter. The volume of water in the calorimeter was 100 cm*3. The increase in temperature recorded was 15.7 °C
4.18 J of energy are needed to increase the temperature of 1cm*3 of water by 1°C
Use this information to calculate the heat energy released in KJ per g of biomass. Show your working. (3 marks)

What are you thinking so far?

Reply 2

Frashostra.

3

We can use the formula Q = m * c * ΔT to calculate the heat energy released in kJ per gram of biomass, where Q is the heat energy, m is the mass of biomass, c is the specific heat capacity of water (which is 4.18 J/g°C), and ΔT is the change in temperature.

First, let's calculate the total heat energy released by the combustion of 2g of biomass:

Q = m * c * ΔT
Q = 2g * 4.18 J/g°C * 15.7°C
Q = 131.99 J

Now, let's convert the answer to kJ per gram of biomass:

Q per gram of biomass = Q / m
Q per gram of biomass = 131.99 J / 2g
Q per gram of biomass = 65.995 J/g

Finally, we can convert the answer to kJ/g by dividing by 1000:

Q per gram of biomass in kJ/g = 65.995 J/g / 1000
Q per gram of biomass in kJ/g = 0.066 kJ/g

Therefore, the heat energy released in kJ per gram of biomass is 0.066 kJ/g.

Original post by Laychips

Hi, can anyone please help me with this question?

A 2g sample of biomass was fully combusted in a calorimeter. The volume of water in the calorimeter was 100 cm*3. The increase in temperature recorded was 15.7 °C
4.18 J of energy are needed to increase the temperature of 1cm*3 of water by 1°C
Use this information to calculate the heat energy released in KJ per g of biomass. Show your working. (3 marks)

Reply 3

hafsahr

Original post by Laychips

Hi, can anyone please help me with this question?

A 2g sample of biomass was fully combusted in a calorimeter. The volume of water in the calorimeter was 100 cm*3. The increase in temperature recorded was 15.7 °C
4.18 J of energy are needed to increase the temperature of 1cm*3 of water by 1°C
Use this information to calculate the heat energy released in KJ per g of biomass. Show your working. (3 marks)

Hi,

My approach is slightly different to the method given by the other user and I also get a different answer. Looking at the mark scheme however, the correct answer is 3.28 which I get. Biology is definitely my most weakest subject so please excuse the bad explanation. I hardly understand it myself.

Firstly I used q=mcΔT
The unit for q would be J, unit for mass is grams, unit for c is J and ΔT can be celcius or kelvin but in biology A level we are not expected to know Kelvin.

I then consider that 4.18 J is required to increase the temperature of 1cm^3 of water by 1 celcius as given in the question. It also mentions the volume of water is 100cm^3 so I times the value of 4.18 by 100. (I'm unsure if my way is right but I thought I'd share it anyway)

Then I plug in = 2* 418* 15.7 which equals 13125.2 J, then I divide this by 1000 to get 13.1252 kJ and divide that by 2 grams to get 6.5626 kJ g^-1. (HOWEVER THIS ANSWER IS INCORRECT AND ONLY AWARDS YOU 1 MARK) but I made an observation that dividing 6.56 by 2 gives you 3.28 which is indeed the correct answer in the mark scheme.

I'm actually beyond confused why this is the case but I guess examiners don't really need to know that tehehe. I hope this helped even though it was very similar to what the other user also did.

Reply 4

chan432

9

Original post by Laychips

Hi, can anyone please help me with this question?

A 2g sample of biomass was fully combusted in a calorimeter. The volume of water in the calorimeter was 100 cm*3. The increase in temperature recorded was 15.7 °C
4.18 J of energy are needed to increase the temperature of 1cm*3 of water by 1°C
Use this information to calculate the heat energy released in KJ per g of biomass. Show your working. (3 marks)

Might be a bit of a late response but I'll jus say it for anyone else who's struggling.
So you do Q=mcT
which means Q=100 x 4.18 x 15.7
you would use 100 as the mass because it talks about the specific heat capacity of the water (4.18) and 100cm3 is the volume of water (100cm3=100g)
the answer would be 6562.6J which would then need to be converted into kJ so would become 6.5626kJ
and then because you can see the units are kJ/g you need to do 6.5626/2 (because the mass of the sample is 2) and the answer is 3.28 (3sf)

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