You are Here: Home

# Calculation of molecular formula of hydrocarbon (REPOST) watch

1. Here, the temperature is kept constant, so we can consider the volume ratio the same as the mole ratio.
Now I want to know if my method of approach is correct or not.

Potassium hydroxide absorbs all the CO2, so we had 40 cm^3 of CO2, and therefore, X= 40

Now we can develop an equation

total volume - 20 cm^3 = volume of CO2 and H2O
(10 + 40 + y/4) - 20 = 40 + y/2
30 + y/4 = 40 + y/2
-10 = y/4
-40 = y

Now we can divide both by 10 to get:
x=4, y=-4
Therefore CxHy = C4H4

Now the problem is that why I got a negative 4?
Is my approach correct?

I have actually found a similar question answered somewhere and here is it:

My question is that why the water volume was not taken into account here?

Your responses will be much appreciated
Attached Images

2. (Original post by Daniel Atieh)

Here, the temperature is kept constant, so we can consider the volume ratio the same as the mole ratio.
Now I want to know if my method of approach is correct or not.

Potassium hydroxide absorbs all the CO2, so we had 40 cm^3 of CO2, and therefore, X= 40

Now we can develop an equation

total volume - 20 cm^3 = volume of CO2 and H2O
(10 + 40 + y/4) - 20 = 40 + y/2
30 + y/4 = 40 + y/2
-10 = y/4
-40 = y

Now we can divide both by 10 to get:
x=4, y=-4
Therefore CxHy = C4H4

Now the problem is that why I got a negative 4?
Is my approach correct?

I have actually found a similar question answered somewhere and here is it:

My question is that why the water volume was not taken into account here?

Your responses will be much appreciated
If you measure things at room temperature the water is a liquid and hence does not have a gas volume ...
3. (Original post by charco)
If you measure things at room temperature the water is a liquid and hence does not have a gas volume ...
Perfectly clear.

But now this causes more problems to the above method I followed. Is it correct? I accounted for the volume of water/moles of water in my calculation, is that okay?

Posted from TSR Mobile
4. (Original post by Daniel Atieh)
Perfectly clear.

But now this causes more problems to the above method I followed. Is it correct? I accounted for the volume of water/moles of water in my calculation, is that okay?

Posted from TSR Mobile
No, it's not ok if the measurements are taken at room temperature. You have to assume that the volume of water(g) is zero...
5. (Original post by charco)
No, it's not ok if the measurements are taken at room temperature. You have to assume that the volume of water(g) is zero...
Charco, I am genuinely thankful to you. Sorry for one more stupid question, but I needed it!

TSR Support Team

We have a brilliant team of more than 60 Support Team members looking after discussions on The Student Room, helping to make it a fun, safe and useful place to hang out.

This forum is supported by:
Updated: January 23, 2015
Today on TSR

### The most controversial member on TSR?

Who do you think it is...

### Is confrontation required?

Discussions on TSR

• Latest
Poll
Discussions on TSR

• Latest

The Student Room, Get Revising and Marked by Teachers are trading names of The Student Room Group Ltd.

Register Number: 04666380 (England and Wales), VAT No. 806 8067 22 Registered Office: International House, Queens Road, Brighton, BN1 3XE