Question

ordinary water contains one part of heavy water per 6000 parts by weight . the no. of heavy water molecules present in a drop of water of volume 0.01mL is? (density of water = 1/mL) (ans :5 x 10¹⁶)

Answer 1

$$\begin{gathered} Ordinarywatercontainsonepartofheavywaterper6000partsbyweight. \\ Thatmeans6000gofordinarywatercontains1gofheavywater. \\ Thegivenmassofordinarywater=volume\times density=0.01\times 1=0.01g. \\ So0.01gofordinarywatercontains=\frac{1}{6000}\times 0.01=1.67\times {10}^{-6}gofheavywater \\ Noofmolesofheavywater=\frac{massoftheheavywater}{molarmassof{D}_{2}O(4+16)}=\frac{1.67\times {10}^{-6}}{20}=0.0835\times {10}^{-6}mol \\ Noofmolecules=mole\times avogadronumber \\ =0.0835\times {10}^{-6}\times 6.022\times {10}^{23} \\ =0.5028\times {10}^{17} \\ =5.028\times {10}^{16}molecules \end{gathered}$$