Question 14 of 26 Map a Sapling Learning macmillan The concept of thermionic emi

Question

Question 14 of 26 Map a Sapling Learning macmillan 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 9.50 x 10 7 C. How many electrons, ne, has the filament lost? If the filament is 2.00 g of tungsten, what fraction of electrons, ef, 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 x 23 1/mol.) Number Number

Explanation / Answer

We know that

The number of electrons is given by Q =ne

n =Q/e =9.50*10-7/1.6*10-19 =5.93*1012

Now we have to find the number of electrons present in 2g of tungsten

Number of the moles in the tungsten is =mass/Molecular weight =2/183.8 =0.01088mol

Now the number of atoms present is =6.02*1023*0.01088 =0.0654*1023

and now the total number of electrons is 0.0654*1023*74 =4.846*1023

Therefore the fraction of electron present is

ne =5.93*1012/4.846*1023=1.22*10-11electrons

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