Question

A graphite pencil weighs $3.0\times {10}^{-10}g.$ How many carbon atoms are present in it? $(N_A=6\times {10}^{23})$

• A. $1.5\times {10}^{13}$
• B. $5\times {10}^{12}$
• C. $2\times {10}^{33}$
• D. $1.5\times {10}^{10}$

Answer 1

Answer: (A)

$1.5\times {10}^{13}$

Given mass of C, $w=3\times {10}^{-10}g$

Molar mass of C, $M=12g/mol$

Moles of C, $n=\frac{w}{M}=\frac{3\times {10}^{-10}}{12}=2.5\times {10}^{-11}mol$

Number of C atoms, $N=n\times N_A=2.5\times {10}^{-11}\times 6\times {10}^{23}=1.5\times {10}^{13}atoms$