Question

Number of atoms of $He$ in $100\, u$ of $He$ (atomic wt of $He$ is $4$) are

• A. 25
• B. 100
• C. 50
• D. $100\times 6\times 10^{-23}$

Answer 1

Answer: (A)

25

Answer 2

Number of atoms of a substance $=$Number of gram molecular weight $\times 6.02 \times 10^{23} \times$ atomicity $=\frac{\text { weight of substance }}{\text { number of gram molecules }} \times 6.02 \times 10^{23} \times$ atomicity $\left(\because 1 u=1.6 \times 10^{-24} \,g\right.$ $\left.100 \,u=1.6 \times 100 \times 10^{-24}\, g\right)$ $=\frac{100 \times 1.6 \times 10^{-24}}{4} \times 6.023 \times 10^{23} \times 1$ $=25 \times 1.6 \times 10^{-24} \times 6.023 \times 10^{23}$ $=25$