# Define a molecule. How many molecules are present in

1.\[9\] grams of water

2.\[17\] grams of ammonia

Answer

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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.

**Complete answer:**

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.

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