Calculation of mass from Avogadro's number — lesson. Science State Board, Class 10.

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1.	Calculate the mass of $1.51\times10^{23}$ molecules of water.

Data:

Given molecules of

$H_2O=1.51\times10^{23}$

Molecular mass of

$H_2O=18$

Avogadro's number

$=6.023\times10^{23}$

Solution:

$\begin{array}{l} Number of moles & = & \frac{Number of molecules of water}{Avogadro number} \\ = & \frac{1.51 \times 10^{23}}{6.023 \times 10^{23}} \\ = & \frac{1}{4} \\ = & 0.25 mole \end{array}$

$\begin{array}{l} Number of moles & = & \frac{Mass}{Molecular mass} \\ 0.25 & = & \frac{Mass}{18} \\ Mass & = & 0.25 \times 18 \\ Mass & = & 4.5 g \end{array}$

Hence, the mass of

$1.51\times10^{23}$ molecules of water

$=4.5$

$g$

2.	Calculate the mass of $5\times10^{23}$ molecules of glucose.

Data:

Given molecules of glucose

$=1.51\times10^{23}$

Molecular mass of glucose

$=180$

Avogadro's number

$=6.023\times10^{23}$

Formula:

$\begin{array}{l} Mass of glucose & = & \frac{Molecular mass \times Number of particles}{Avogadro number} \\ = & \frac{180 \times 5 \times 10^{23}}{6.023 {\times 10}^{23}} \\ = & 149.43 g \end{array}$

Hence, the mass of

$5\times10^{23}$ molecules of glucose

$=149.43$

$g$

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5. Calculation of mass from Avogadro's number

Theory: