The largest number of molecules is in:
(a) 36g of water
(b) 28g of carbon monoxide
(c) 46g of ethyl alcohol
(d) 54g of nitrogen pentoxide

Hint: We can use the formula to calculate the number of moles by taking Avogadro’s number, molecular weight of the compound and its given weight.
\[\text{Number of moles = }\dfrac{\text{weight}}{\text{molar mass}}\text{ x Avogadro }\!\!'\!\!\text{ s number}\]

Complete answer:
The formula to calculate the number of moles of a given substance is as follows.
\[\text{Number of moles = }\dfrac{\text{weight}}{\text{molar mass}}\text{ x Avogadro }\!\!'\!\!\text{ s number}\]
Avogadro’s Number = \[\text{6}\text{.023 x 1}{{\text{0}}^{\text{23}}}\]
We have to calculate the number of molecules in the given options

Number of molecules in 36g of water
Chemical formula of water = ${{\text{H}}_{\text{2}}}\text{O}$
Molecular weight of water = 18g
Given weight of water = 36g
Number of moles = $\dfrac{36}{18}\text{x 6}\text{.023 x 1}{{\text{0}}^{\text{23}}}\text{= 2 x 6}\text{.023 x 1}{{\text{0}}^{\text{23}}}$

Number of molecules in 28g of carbon monoxide
Chemical formula of carbon monoxide = $\text{CO}$
Molecular weight of carbon monoxide = 28g
Given weight of carbon monoxide = 28g
Number of moles = $\dfrac{28}{28}\text{x 6}\text{.023 x 1}{{\text{0}}^{\text{23}}}\text{= 1 x 6}\text{.023 x 1}{{\text{0}}^{\text{23}}}$

Number of molecules in 46g of ethyl alcohol
Chemical formula of ethyl alcohol = ${{\text{C}}_{\text{2}}}{{\text{H}}_{\text{5}}}\text{OH}$
Molecular weight of ethyl alcohol = 46g
Given weight of ethyl alcohol =46g
Number of moles = $\dfrac{46}{46}\text{x 6}\text{.023 x 1}{{\text{0}}^{\text{23}}}\text{= 1 x 6}\text{.023 x 1}{{\text{0}}^{\text{23}}}$

Number of molecules in 54g of nitrogen pentoxide
Chemical formula of nitrogen pentoxide = ${{\text{N}}_{\text{2}}}{{\text{O}}_{\text{5}}}$
Molecular weight of nitrogen pentoxide = 108g
Given weight of nitrogen pentoxide = 54g
Number of moles = $\dfrac{54}{108}\text{x 6}\text{.023 x 1}{{\text{0}}^{\text{23}}}\text{= 0}\text{.5 x 6}\text{.023 x 1}{{\text{0}}^{\text{23}}}$
- From the above calculations we can say that 36 g of water contains the highest number of molecules when compared to remaining molecules given in the options.

Therefore, the correct answer is option (a).

Note:
We should know the molecular weight of the chemicals to calculate the number of moles from it. The molecular weight of the compound = sum of the molecular weights of each element involved in the chemical composition.