Mole Concept And Avogadro's Number

Question

Calculate the number of molecules present in 9 g of water ($\text{H}_2\text{O}$). ($N_A = 6.022 \times 10^{23}$ mol$^{-1}$, Molar mass of water = 18 g/mol)

RankGuru · Accepted Answer

Avogadro's number specifies that one mole of any substance contains exactly $6.022 \times 10^{23}$ entities. The number of molecules is found by first converting mass to moles, then multiplying by Avogadro's number. Moles of water $= 9/18 = 0.5$ mol. Number of molecules $= 0.5 \times 6.022 \times 10^{23} = 3.011 \times 10^{23}$. Option $1.806 \times 10^{23}$ is incorrect; it would correspond to $0.3$ mol, not 0.5 mol. Option $6.022 \times 10^{23}$ is the number for a full mole (18 g), not 9 g. Option $1.204 \times 10^{24}$ corresponds to 2 mol (36 g of water), which is twice the given mass. This question directly applies the NCERT concept linking mass, moles, and number of particles via Avogadro's number. Plausibility check: 9 g is exactly half of 18 g (one mole), so the answer must be exactly half of $N_A$, giving $3.011 \times 10^{23}$.

Mole Concept And Avogadro's Number

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