[JFaithx]

Hi I'm stuck with this past paper question on benzene.

Napthalene C10H8 has the following structure (pic of napthalene, basically two benzene rings stuck together)
The number of moles of hydrogen gas required for complete hydrogenation of 12.8g napthalene will be?

The answer is supposed to be 0.5 but I keep getting 0.4. Anyone help?

[Friar Chris]

(Original post by JFaithx)
Hi I'm stuck with this past paper question on benzene.

Napthalene C10H8 has the following structure (pic of napthalene, basically two benzene rings stuck together)
The number of moles of hydrogen gas required for complete hydrogenation of 12.8g napthalene will be?

The answer is supposed to be 0.5 but I keep getting 0.4. Anyone help?

WAIT, I think I worked out the obvious blunder (in my previous post)

We're not forming tetrahydronaphthalene, we're forming decahydronapthalene, C₁₀H₁₈.


My initial post was half correct, and the answer is indeed 0.5 - we are hydrogenating all the π bonds and adding hydrogen - there are 5 bonds and thus we need 10 hydrogen atoms as hydrogenating 1 π bond produces two σ bonds to hydrogens...

Molecular mass of naphthalene is 128, so 12.8 is 0.1mol, therefore 0.5mol of hydrogen MOLECULES are needed because it is of course in the form of H₂ - full hydrogenation requires 10 hydrogen atoms per molecule of naphthalene, so 5 times as many molecules of hydrogen are required thus 0.1*5 = 0.5...

Fixed


I'm guessing to get 0.4 you were trying to only partially hydrogenate it to tetrahydronaphthalene ('tetralin') removing two π bonds from one of the rings and adding 4 hydrogens... The trick was hydrogen exists as H₂, of course, so you need half as many moles of hydrogen molecules as you do hydrogen atoms, which confounded you twicefold?