Calculation based on number of molecules — lesson. Science State Board, Class 10.

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1.	Calculate the number of molecules in $11.2$ $L$ of $CO_2$ at STP.

Data:

Given volume of

$CO_2=11.2$

$L$

Molar volume of

$CO_2=22.4$

$L$

Avogadro's number

$=6.023\times10^{23}$

Solution:

$\begin{array}{l} Number of moles of {CO}_{2} & = & \frac{Volume at STP}{Molar volume} \\ = & \frac{11.2}{22.4} \\ = & 0.5 mole \end{array}$

$\begin{array}{l} Number of molecules of {CO}_{2} & = & Number of moles of {CO}_{2} \times Avogadro number \\ = & 0.5 \times 6.023 \times 10^{23} \\ = & 3.011 \times 10^{23} molecules \end{array}$

Therefore, the number of molecules in

$11.2$

$L$ of

$CO_2$ at STP

$=3.011\times10^{23}$

2.	Calculate the number of molecules in $54$ $g$ of $H_2O$ .

Data:

Given mass of

$H_2O=54$

$g$

Gram molecular mass of

$H_2O=18$

$g$

Avogadro's number

$=6.023\times10^{23}$

Solution:

$\begin{array}{l} Number of molecules = \frac{Avogadro number \times Given mass}{Gram molecular mass} \\ \begin{array}{l} Number of molecules of water & = & 6.023 \times 10^{23} \times \frac{54}{18} \\ = & 18.069 \times 10^{23} \end{array} \end{array}$

Therefore, the number of molecules in

$54$

$g$ of

$H_2O=18.069\times10^{23}$

$molecules$

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10. Calculation based on number of molecules

Theory: