The mass of a carbon-12 atom was determined by a mass spectrometer and found to be equal to $1.992648 \times 10^{-23} \mathrm{~g}$. It is known that 1 mole of $\mathrm{C}-12$ atom weighing 12 g contains $\mathrm{N}_{\mathrm{A}}$ number of atoms. Thus,

1 mole of $\mathrm{C}-12$ atoms $=12 \mathrm{~g}=6.022 \times 10^{23}$ atoms

$\Rightarrow 6.022 \times 10^{23}$ atoms of C - 12 have mass $=12 \mathrm{~g}$

$\therefore \quad 1$ atom of $\mathrm{C}-12$ will have mass $=\frac{12}{6.022 \times 10^{23}} \mathrm{~g}$

$=1.992648 \times 10^{-23} \mathrm{~g} \approx 1.99 \times 10^{-23} \mathrm{~g}$

Least precise term 2.5 or 3.5 has two significant figures.

Hence, the answer should have two significant figures

$$\frac{2.5 \times 1.25 \times 3.5}{2.01} \approx 5.4415=5.4$$

Symbol for SI unit of mole is mol.

One mole is defined as the amount of a substance that contains as many particles and there are atoms in exactly $12 \mathrm{~g}(0.012 \mathrm{~kg})$ of the ${ }^{12} \mathrm{C}$ - isotope.

$$\frac{1}{12} \mathrm{~g} \text { of }{ }^{12} \mathrm{C} \text {-isotope }=1 \mathrm{~mole}$$

Molality It is defined as the number of moles of solute dissolved in 1 kg of solvent. It is independent of temperature.

Molarity It is defined as the number of moles of solute dissolved in 1 L of solution. It depends upon temperature (because, volume of solution $\propto$ temperature).