Calculating number of moles from mass and volume — lesson. Science State Board, Class 10.

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1.	Calculate the number of moles in \(46\) \(g\) of sodium.

Data:

Given mass of sodium \(= 46\) \(g\)

Atomic mass or mass number of sodium \(= 23\)

Formula:

The formula to find the number of moles from the mass is,

Number of moles = \frac{Mass of the element}{Atomic mass of the element}

Solution:

By substituting the values in the above formula, we get,

\begin{array}{l} Number of moles & = & \frac{46}{23} \\ = & 2 \end{array}

Hence, the number of moles in \(46\) \(g\) of sodium \(=2\) \(moles\)

2.	Calculate the number of moles in \(5.6\) \(L\) of oxygen at STP.

Data:

Given volume of oxygen \(= 5.6\) \(L\)

Molar volume of oxygen at STP \(= 22.4\) \(L\)

Formula:

The formula to find the number of moles from the volume is,

Number of moles = \frac{Given volume of O_{2} at STP}{Molar volume at STP}

Solution:

By substituting the values in the above formula, we get,

\begin{array}{l} Number of moles & = & \frac{5.6}{22.4} \\ = & 0.25 moles \end{array}

Hence, the number of moles in \(5.6\) \(L\) of oxygen at STP\(=0.25\) \(moles\)

3.	Calculate the number of moles of a sample that contains \(12.046\times10^{23}\) atoms of iron.

Data:

Given the number of atoms of iron \(=12.046\times10^{23}\)

Avogadro's number \(=6.023\times10^{23}\)

Formula:

Number of moles = \frac{Number of atoms of iron}{Avogadro number}

Solution:

\begin{array}{l} Number of moles & = & \frac{12.046 \times 10^{23}}{6.023 \times 10^{23}} \\ = & 2 \end{array}

Hence, the number of moles of a sample that contains \(12.046\times10^{23}\) atoms of iron\(=2\) \(moles\)

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3. Calculating number of moles from mass and volume