Question

A water sample contains 0.10 ppm of chloroform. How many molecules of chloroform will be present in 0.5 mL of this water?

• A. $4.18\times {10}^{-5}\times N_A$
• B. $4.18\times {10}^{-6}\times N_A$
• C. $4.18\times {10}^{-10}\times N_A$
• D. $\mathrm{4..18}\times {10}^{-9}\times N_A$

Answer 1

The correct option is C $4.18\times {10}^{-10}\times N_A$

0.5 mL≡0.5 g of water
${10}^6$ g of water contains 0.10 g $CHC{l}_3$.
0.5 g of water contains $\frac{0.1}{{10}^6}\times 0.5$ g of $CHC{l}_3$.
Number of moles of $CHC{l}_3$ present in the sample,
$n_{CHC{l}_3}=\frac{0.1}{{10}^6}\times \frac{0.5}{119.5}=4.18\times {10}^{-10}$
Number of molecules of $CHC{l}_3$
$4.18\times {10}^{-10}\times N_A$