Question

What is the mass of one molecule of yellow phosphorus? (Atomic mass, $P=30$)

• A. $1.993\times {10}^{-22}kg$
• B. $1.993\times {10}^{-19}mg$
• C. $4.983\times {10}^{-20}mg$
• D. $4.983\times {10}^{-23}kg$

Answer 1

Answer: (B)

$1.993\times {10}^{-19}mg$

Yellow phosphorus is an impure form of white phosphorus $\left(P_4\right)$.

Molecular mass of yellow phosphorus $\left(P_4\right)=4\times 30=120$

$\therefore$ According to Avogadro's hypothesis,

Mass of $6.022\times {10}^{23}$ molecules $=120g$

Mass of 1 molecule $=\frac{120\times 1}{6.022\times {10}^{23}}$

$=19.926\times {10}^{-23}g=1.993\times {10}^{-22}g$

$=1.993\times {10}^{-19}mg$

Hence, option B is correct.