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### 1B.9 help

Posted: **Sun Oct 20, 2019 8:39 pm**

by **Mallika Singh 1G**

A lamp rated at 32 W emits violet light of wavelength 420 nm. How many photons of violet light can the lamp generate in 2.0 s? How many moles of photons are emitted in that time interval?

What formula would we use for this? (this is probably in the wrong forum) Thank you!

### Re: 1B.9 help

Posted: **Sun Oct 20, 2019 8:41 pm**

by **Elizabeth Harty 1A**

You would use the equation E=hcλ and then you would multiply 32 by 2 because it is 2 seconds. You would then use the molar equation to get the number of moles of photons.

### Re: 1B.9 help

Posted: **Sun Oct 20, 2019 8:43 pm**

by **Christine Honda 2I**

You need to convert nm to m then use the equation:

E=hc/lambda.

E = [(6.626 x 10^-34 Js)(3.0 x 10^8 m/s)]/(420 x 10^-9 m) = 4.7 x 10^-19 J

Doing this gives us how much energy in Joules each photon has.

Then multiply 32 W by 2, so 64 J are generated in 2.0 seconds. Use the answer we found above to get the number of photons generated.

# photons = 64 J x (1 photon/4.7 x 10^-9 J) = 1.4 x 10^20 photons

Then use 6.022 x 10^23 photons/ mol as a conversion factor

### Re: 1B.9 help

Posted: **Sun Oct 20, 2019 8:47 pm**

by **Julie Park 1G**

You would first convert the wavelength of 420nm to meters

Then use E=hc/lamba to find the energy in J/photon

Then find the number of photons by dividing the energy from 64J

Then find the moles of photons by multiplying the number of photons by (1mol/6.022x10^23 photons)

### Re: 1B.9 help

Posted: **Sun Oct 20, 2019 9:17 pm**

by **Varadhan_3G**

A lamp rated at 32 W emits violet light of wavelength 420 nm. How many photons of violet light can the lamp generate in 2.0 s? How many moles of photons are emitted in that time interval?

A watt is a unit of power, 1.00 J/s;

we can rewrite 32 W as 32 J/s, so in 2 seconds: (32J/s)*(2s/1), the seconds cancel out to give you 64J.

Also, convert wavelength from nm to m: 420 x 10^-9 m

First, using the equation: e=hc/λ, we have to find the energy in Joules (per photon) in light of a wavelength 420nm.

e= [(6.626 x 10^-34 Js)(3.0 x 10^8 m/s)]/(420 x 10^-9 m) (meters and seconds cancel out)

e= 4.7 x 10^-19 J/photon

Simply divide 64 J, energy generated in 2 seconds, by the above amount to get the amount of photons in 2.0 seconds: 1.4x10^20 photons.

To find moles of photons, use Avogadro's number:

(1.4x10^20 photons) x (1 mol/6.022 x 10^23 photons) = 2.3*10^-4 mol photons.