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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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