Question

A sample of municipal water contains one part of urea (molecular wt $=60$) per million parts of water by weight. The number of urea molecules in a drop of water of volume $0.05ml$ is

• A. $2.5\times {10}^{14}$
• B. $5\times {10}^{14}$
• C. $5\times {10}^{13}$
• D. $5\times {10}^{15}$

Answer 1

The correct option is B $5\times {10}^{14}$

$1ppm=1mg/L={10}^{-3}g/L$

Water sample contains $1ppm$ urea concentration.

$\therefore {10}^{-3}g$ of urea in $1L$

$xg$ urea in $0.05\times {10}^{-3}L$

$x=0.05\times {10}^{-6}g$

$60g$ urea $=6.023\times {10}^{23}molecules$

$0.05\times {10}^{-6}g=nmolecules$

$n=\frac{6.023\times {10}^{23}\times 0.05\times {10}^{-6}}{60}$

$=0.005\times {10}^{17}$

$=5\times {10}^{14}molecules$