Question

Which of the following represents the least number of molecules?

• A. $20\text{g of}{H}_{2}O\text{(Molar mass}=18.02\text{g/mol})$
• B. $77\text{g of}C{H}_4\text{(Molar mass}=16.06\text{g/mol})$
• C. $68\text{g of}Ca{H}_2\text{(Molar mass}=42.09\text{g/mol})$
• D. $100\text{g of}{N}_{2}O\text{(Molar mass}=44.02\text{g/mol})$

Answer 1

The correct option is A $20\text{g of}{H}_{2}O\text{(Molar mass}=18.02\text{g/mol})$

1 mole of a substances will have $N_A$ number of molecules, so the molecule with the least number of moles will have the least number of molecules.
$\text{Number of moles}=\frac{\text{given mass}}{\text{molar mass}}$

$\text{For}20\text{g}{H}_{2}O,\text{number of moles}=\frac{20}{18}=1.11\text{moles}$
$\text{For}77\text{g}C{H}_4,\text{number of moles}=\frac{77}{16}=4.81\text{moles}$
$\text{For}60\text{g}Ca{H}_2,\text{number of moles}=\frac{68}{42}=1.62\text{moles}$
$\text{For}100\text{g}{N}_{2}O,\text{number of moles}=\frac{100}{44}=2.27\text{moles}$
As water has the least number of moles, hence it will have the least number of molecules.