Question

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

Answer 1

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.
$$\text{Number of moles = }{\displaystyle \frac{\text{weight}}{\text{molar mass}}}\text{ x Avogadro }{}^{\prime }\text{ s number}$$

Complete answer:
The formula to calculate the number of moles of a given substance is as follows.
$$\text{Number of moles = }{\displaystyle \frac{\text{weight}}{\text{molar mass}}}\text{ x Avogadro }{}^{\prime }\text{ s number}$$
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 = ${\displaystyle \frac{36}{18}}\text{x 6}\text{.023 x 1}{\text{0}}^{\text{23}}\text{= 2 x 6}\text{.023 x 1}{\text{0}}^{\text{23}}$

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 = ${\displaystyle \frac{28}{28}}\text{x 6}\text{.023 x 1}{\text{0}}^{\text{23}}\text{= 1 x 6}\text{.023 x 1}{\text{0}}^{\text{23}}$

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 = ${\displaystyle \frac{46}{46}}\text{x 6}\text{.023 x 1}{\text{0}}^{\text{23}}\text{= 1 x 6}\text{.023 x 1}{\text{0}}^{\text{23}}$

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 = ${\displaystyle \frac{54}{108}}\text{x 6}\text{.023 x 1}{\text{0}}^{\text{23}}\text{= 0}\text{.5 x 6}\text{.023 x 1}{\text{0}}^{\text{23}}$
- 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.