Ncert Exercises & Solutions

22. Express unified atomic mass unit in kg.

Hint: One atomic mass unit is 1/12 of the mass of a

{}_{6}C_{12}

atom.

Step 1: Find the mass of one

{}_{6}C_{12}

atom.

Mass of one mole of ${}_{6}C_{12}$ atom= 12g
Number of atoms in one mole = Avogadro's number
$= 6 .023 \times 10^{23}$

$∴ {Mass of one}_{6} C^{12} atom = \frac{12}{6 .023 \times 10^{23}} g$
$Step 2 : Find the value of 1 amu in kg .$
$1 amu = \frac{1}{12} \times {mass of one}_{6} C^{12} atom$
$∴ 1 amu = (\frac{1}{12} \times \frac{12}{6 .023 \times 10^{23}}) g = 1 .67 \times 10^{- 24} g$
$= 1 .67 \times 10^{- 27} kg (∵ 1 g = 10^{- 3} kg)$

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