ore3/30/2018 10:55 PM76.6/1002/26/2018 07:28 AM Gradebook Print E Calculator -1

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ore3/30/2018 10:55 PM76.6/1002/26/2018 07:28 AM Gradebook Print E Calculator -1 Periodic Table 4, Question t- 21 of 30 Mapdeb Sapling Learning The concept of thermionic emission is the basis of modern X-ray tubes, which are the workhorse of a medical X-ray image. In this process a filament (a piece of metal in a vacuum) is heated by the application of electrical current. The heat (energy) is transferred to the free' electrons of the metal which allow some electrons to escape from the surface. After a short period of operation the filament has lost enough electrons to give it an overall charge of 1.100 x106 C. How many electrons, ne, has the filament lost? If the filament is 1.40 g of tungsten, what fraction of electrons, e, does this represent? (HINT: The atomic number of tungsten is 74.0, the atomic weight is 183.8 g/mol and Avogadro's constant-6.02% 1021/mol.) Number 6.88 x 10 12 Number e1.42x 10 3 There is additional feedback available View this feedback by clicking on the bottom divider bar. Click on the divider Incorrect. bar again to hide the additional feedback. Close Gou Solution d Try Again Next

Explanation / Answer

a)
Consider n be the total number of electrons emitted.
ne = q
Where e is the charge of a single electron and q is the net charge of the filament
n = q/e
= (1.100 x 10-6) / (1.602 x 10-19)
= 6.866 x 1012

b)
Each atom of tungsten (W) gives 2 electrons for thermionic emission.
183.8 g of W contains Avogadro's number of W atoms, ie 6.02 x 1023.
Number of atoms in1.4 g = (1.4/183.8) x (6.02 x 1023)
= 4.585 x 1021
Number of electrons in 1.4 g W, N = 2 x 4.585 x 1021
= 9.171 x 1021

Fraction of emitted electrons = n/N
= (6.866 x 1012) / (9.171 x 1021)
= 7.487 x 10-10

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