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E. A student used a carbon pencil to write his homework. The mass of this was found to be 5 mg. With the help of this calculate.

a. The number of moles of carbon in his homework writing

b. The number of carbon atoms in 12 mg of his homework writting
in Chapter 1: Some Basic Concepts of Chemistry by Expert (10.9k points)
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Given : 

mass of carbon = 5mg =\(5\times 10^{-3} g\) 

Atomic mass of carbon=12u 

Solution ::

a. The number of moles of carbon in his homework writing

 number of moles=\(mass \space of \space substance \over molar \space mass \space of \space substance\)  

=\(5\times 10^{-3} g\over 12 g\space mol^{-1}\) 

= 4.167\(\times 10^{-4} mol\) 

b. The number of carbon atoms in 12 mg of his homework writting

mass of carbon =12mg \(=12\times 10^{-3} g\) 

number of moles =\(mass \space of \space substance \over molar \space mass \space of \space substance\) 

=\(12\times 10^{-3} g\over 12 g\space mol^{-1}\) 

=1\(\times 10^{-3}\) mol 

\(\therefore \) number of atoms of carbon =number of moles\(\times \) avogadros constant

=\(1\times 10^{-3}\times 6.022\times10^{23}\) 

=\(6.022\times 10^{20}\)  atoms

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