Question

Which of the following would contain the same number of atoms as $20g$ of calcium? [At. masses: $Ca=40,Mg=24,C=12$ ]

• A. 24 g of magnesium
• B. 12 got carbon
• C. 24 got carbon
• D. 12 g of magnesium

Answer 1

Answer: (D)

$\because$ Number of atoms in $40g$ of
$Ca=6.022\times 10^{23}$ (Avogadro's number)
$\therefore$ Number of atoms in $20g$ of
$Ca=\frac{6.022\times 10^{23}\times 20}{40}=3.011\times 10^{23}$
Like this $\because$ Number of atoms in $24g$ of
$Mg=6.022\times 10^{23}$
$\therefore$ Number of atoms in $12g$ of
$Mg=\frac{6.022\times 10^{23}\times 12}{24}=3.011\times 10^{23}$
Like this $\because$ Number of atoms in $24g$ of
$Mg=6.022\times 10^{23}$
$\therefore$ Number of atoms in $12g$ of $Mg$
$=\frac{6.022\times 10^{23}\times 12}{24}=3.011\times 10^{23}$