Question

Calculate the no. of atoms of $Au$ in $1$ gm of $Au$. No. of molecule of $H_{2}O$ in a drop of $H_{2}O$ weighting $0.05$ gm, (Atomic mass of $Au$ is $197$).

Answer 1

a) Gram atomic mass of $Au=197g$

197 g Au contains $=6022\times {10}^{23}$ atoms

$\therefore$ 1 gm of Au contains $=\frac{6022\times {10}^{23}}{197}=306\times {10}^{23}$ atoms

b) molecule mass of water = 18 g

no of molecules in 0.05 g of water $=\frac{0.05}{18}=278\times {10}^{-3}$ molecules

one molecule $=6023\times {10}^{22}$ atoms/ molecules.

$\therefore 2.78\times {10}^{-3}$ molecules of water contains $=(278\times {10}^{-3})$

$(6023\times {10}^{23})$

$=1.67\times {10}^{21}$ molecules