Jim's maths and physics problems

[JimC]

Thanks, lpetrich - I have been a little busy to respond before...

Your problem and solution on pH was a good "re-teaching" tool for me...

Will work on the other one soon, but one thing - I am pretty certain that you can't simply treat all long chain hydrocarbons (alkanes, I presume you mean) the same. As the molecules get larger, the absolute proportion of carbon atoms increases...

I have an Excel sheet which will sort out the stoichiometry of the combustion of alkanes...

[JimC]

OK, here are my answers:

CH4 3.99 grams

C 2.66 grams

H2 7.94 grams

C9H20 3.49 grams

C21H44 3.45 grams

OK, certainly the long chain alkanes approach CH2, whereupon the grams of oxygen would be 3.42

[Xamonas Chegwé]

The general formula for long chain hydrocarbons is CnH(2n + 2)

The general formula for complete combustion of such a molecule is therefore:

CnH(2n + 2) + (2n + 1) O2 ==> n CO2 + (n + 1) H2O

You can plug the numbers into that if you like.

[JimC]

The general formula for long chain hydrocarbons is CnH(2n + 2)

The general formula for complete combustion of such a molecule is therefore:

CnH(2n + 2) + (2n + 1) O2 ==> n CO2 + (n + 1) H2O

You can plug the numbers into that if you like.

That's what I built an Excel sheet to do!

I know, it's a bit sad...

PS - it is actually (2n + 1) /2 O2 molecules, so when the number of carbon atoms os even, we need this... (example from octane)


2C8H18 + 25O2 ==> 16O2+ 18H2O

In my earliest sheet, I had to have one section for C odd, and another for C even...

[lpetrich]

Good work, JimC. But I should have have been clearer about these hydrocarbons being saturated.

Unsaturated hydrocarbons can have much less hydrogen per carbon atom. Benzene is C6H6, and polycyclic aromatic hydrocarbons can have even less hydrogen.

Turning to common foodstuffs, fats are chemically close to hydrocarbons. Saturated ones are approximately (CH2)n, while unsaturated ones can drop the hydrogen per carbon to about 1.5 (arachidonic acid, with 4 double bonds).

Carbohydrates are close to (CH2O)n, or for polymerized ones, (C6(H2O)5)n.

Proteins are difficult to generalize about, since their component amino acids vary widely. Their structure is (C2H2NOR)n where R is the side chains, with all their variation.