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02:52 Brazilian Center for Research in Physics

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Problem 9

(II) What is the total charge of all the electrons in a 12-kg bar of gold? What is the net charge of the bar? (Gold has 79 electrons per atom and an atomic mass of 197 u.)

$-4.6\times 10^{8} C$ and the net charge is zero

## Discussion

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## Video Transcript

to begin this question with better mind. What is the number off atoms of gold that are in 12 kilograms off gold? For that, we have to use a gratis constant that tells us that in one mole off good, there are 6.2 times 10 to 23 gold atoms, so it gives us follows in one mood. We have 6.2 times stand to 23 atoms before we now have to discover how many rolls off. Good are 12 kilograms. For that, we have to use these information. These information tells us the following. It tells us that one more off gold has 197 grounds. Sure. Now we can do the following one Mode has 197 grounds. Let me converts to kill a rooms by multiplying by 10 to the minus three and now acts. Moons are present in 12 kilograms of gold. Then doing the cross multiplication, we got that 0.197 times X is equals to 12 so acts is equals to 12 divided by 0.197 and this is the number off moles off gold in 12 kilograms off gold. Now we would have this result and use here because now we have 12 divided by 0.197 balls off goat and that number off newly off good has why atoms again do the cross multiplication. To get that why it's equals to 12 divided by 0.1 97 times 6.2 times. Stand to 23. Now to calculate the charge in all electrons, we have to do one thing, which is the following notice that each goat atom has 79 electrons. Therefore, we can say that one at all has 79 electrons, then 12 divided by 0.197 times 6.0 truth time. Stand to 23 atoms has w electrons do the cross multiplication and then the number off electrons is equals to 12 divided by 0.1 and seven times 6.0 Truth I understand to 23 times 79 and this is the number off electrons. And finally remember that the charge off each electron is Stephen by minus 1.602 times stand toe minus 90 problems before finishing the question. Let me organize the board. Okay, so this is the number off electrons in 12 kilograms of good. Now to discover one is the net charge off those electrons Onley. All we have to do is multiply the number off electrons by the charge off each Elektrim and then the charge off all electrons is equals to minus 1.60 Choose time. Stand to my house. 19 times 12 divided by 0.19756 points is your true time, Stan 23 times 79. And these results in a charge off approximately minus 4.6 times. Stand to the eight problems. This is the charge off all electrons contained in 12 kilograms off gold. For the second question, it asks us what is the next charge off news? 12 kilograms of gold. This is a very easy question because a gold atom has zero. That charge because it has 79 electrons and 79 pro tools there for the nets charge often atom zero, which means that the net charge off 12 kilograms off bathrooms. It's outside zero. Therefore, zero is the answer for the second item