[dagitt]

^{Hi, these are some problems I've been working on for a few days now, as review for my midterms...I know we covered all the material in class this first semester, but these are particularly hard! Help for any or all of the questions would be appreciated, thanks}



1. The Watt is the derived SI unit of power, the measure of energy per unit time: 1W=1J/s. A semiconductor laser in a CD player has an output wavelength of 780nm and a power level of 1.12mW. How many photons strike the CD surface during the playing of 74 minutes in length?


2. We can drive off the water from Copper (II) Sulfate pentahydrate by heating. A) How many grams of anhydrous Copper Sulfate could be obtained by heating 173g of hydrated Copper Sulfate? B) If an experiment calls for 2.50moles of anhydrous Copper Sulfate, how many grams of hydrated Copper Sulfate should we use to supply the required amount?


3. A sample of Uranium ore is found to contain 4.64mg of ²³⁸U and 1.22mg of ²⁰⁶Pb. Estimate the age of the ore. The half-life of ²³⁸U is 4.51 * 10⁹ yrs. and the mass of the isotope is 238u.



4. The root mean square speed of molecules of mass m is given by:



Calculate the DeBroglie wavelength of an Argon atom moving with a speed equal to the root mean square speed of argon atoms at 300K. (kb is the Boltzmann's constant which is equal to 1.3807 * 10^-23. J/K)







How do I go about starting these???

[Borek]

1^st - how much energy used while playing? How much energy per one 780nm photon?

2^nd - start with the reaction equation.

3^rd - you are probably to assume original sample didn't contain any Pb.

[ARGOS++]

Dear Dagitt;

To Q.1:

• A.)   One mole Photons of 780nm, also called 1.0 Einstein₍₇₈₀₎, corresponds to a certain amount of Energy, so you can translate 0.00112 J/sec into: mole Photons/sec.
  Therefore see under:  "FAQ: Why Einstein meets Beer-Lambert?”
  (Hint: 1.0 kcal = 4.184 kJoules)
• B.)   Multiply the translated value with the duration of playing in seconds.
  (Remember: 1 mole = 6.02214 * 10²³)
• C.)   That’s all.  (Watch the Units!)

Good Luck!
                    ARGOS⁺⁺