Question

Define a molecule. How many molecules are present in
1.\[9\] grams of water
2.\[17\] grams of ammonia

Answer 1

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.