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4) solve problems

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
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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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