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Last updated date: 03rd Dec 2023
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# Define a molecule. How many molecules are present in1.$9$ grams of water2.$17$ grams of ammonia

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Hint: An atom is considered as the smallest part of an element. We can say that it is a basic ingredient of a common matter. Atom is further divided into three parts i.e., electrons, protons and neutrons.

Molecule: It is a group of two or more atoms chemically bonded together and forms the smallest basic unit of a compound which can participate in a chemical reaction. To find the number of molecules in any compound, we need to multiply the number of moles of that compound with the Avogadro’s constant.
Number of molecules $=$number of moles $\times \,6.023 \times \,{10^{23}}\,\, - (i)$
Number of moles: It is the ratio of given mass of the compound to its molecular mass. The expression to find number of moles is given below:
$n = \dfrac{m}{M}\,\, - (ii)$
Where, $m$is the given mass of the compound and $M$is the molar mass of the given compound.
Number of molecules in $9$ grams of water.
Given mass of water molecule $= 9g$
Molar mass of water molecule $= 18g$
Using equation $(ii)$ for calculating number of moles of water:
Number of moles $n = \dfrac{9}{{18}} \Rightarrow 0.5$
Using equation $(i)$ for calculating the number of molecules of water
Number of molecules $= n \times 6.023 \times {10^{23}}$
$\Rightarrow 0.5 \times 6.023 \times {10^{23}}$
$\Rightarrow 3.01 \times {10^{23}}$
Hence, number of molecules in $9$ grams of water $= 3.01 \times {10^{23}}$molecules.
Number of molecules in $17$ grams of ammonia.
Given mass of ammonia $= 17g$
Molar mass of water molecule $= 17g$
Using equation $(ii)$ for calculating number of moles of water:
Number of moles $n = \dfrac{{17}}{{17}} \Rightarrow 1$
Using equation $(i)$ for calculating the number of molecules of water
Number of molecules $= n \times 6.023 \times {10^{23}}$
$\Rightarrow 1 \times 6.023 \times {10^{23}}$
$\Rightarrow 6.023 \times {10^{23}}$
Hence, the number of molecules in $17$ grams of ammonia $= 6.023 \times {10^{23}}$ molecules.

Note:
Avogadro’s number is the proportionality factor which gives a relation between the number of particles or molecules in a sample when a fixed amount of that sample is taken. Its SI unit is the reciprocal to that of mole.