[Hessenth]

Okay so, I have a test tomorrow and I seem unable to answer any of the practice problems for said subject correctly.

So here's what I do/know. I'll take a problem I can't answer.

Calculate Energy from a Wavelength of 656.3 nm.

I use the equation E = hv , or E = hc/(wavelength) where h = Plank's constant (6.626 x 10^-34 Jxs) and c = speed of light (3.0x10^8)

So I plug it in and get
E = (6.626 x 10^-34)(3.0x10^8)/(656.3x10^-9)

then I end up with: 3.02x10^-19
Answer in the book says: 182.3 kJ/mol

?

I did miss the lecture, so I am obviously missing something. I'm not sure I understand how this all equates to end up in kJ/mol, but it does. All the answers seem similar, and the process seems so simple. Any help would be greatly appreciated.

[Wald_ron]

E=hc/(wavelength)

units for h=Joule*second
units for speed of light= meters*second
wavelengh= meters

(joule*second)(meter/second)/meter = Joules
joules are not equal to kilojoules/mol

[Hessenth]

Ah, I see. However, all the answers are wanted in kJ/mol. how do I get kJ/mol from just Joules?

[Wald_ron]

both J and kJ/mol are different ways of measuring the same thing (Energy) ,

1) convert Joules to kJ

giving you (3.02x10^-22)

2) then multiply by avogardros number for your solution
Don't forget that avogardro's number is reciprocal mols

[Hessenth]

Totally forgot about that!!!! Thanks youu! I forgot av. # = mol^-1

[Borek]

both J and kJ/mol are different ways of measuring the same thing (Energy)

Not exactly. One is just energy, second is energy per mole. You can't say that's the same. It is like stating that distance and speed are the same thing - one is meteres, other is meters per second.

[Wald_ron]

both J and kJ/mol are different ways of measuring the same thing (Energy)

Not exactly. One is just energy, second is energy per mole. You can't say that's the same. It is like stating that distance and speed are the same thing - one is meteres, other is meters per second.

I see, thank you for clearing that up for me, I am currently a student myself.

Are kJ/mol commonly used with Energy Level Diagrams?