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WAEC Chemistry 2015 Paper

Question : 21

Total: 50

[Avogadro's number = 6.02 x 10²³]

Solution:

Concept:

A mole of any gas contains Avogadro’s number of molecules.

The molar mass of oxygen gas

\mathrm{O}_2

32.0\ \text{g mol}^{-1}

Explanation:

Moles of

\mathrm{O}_2

16.0\ \text{g}

\dfrac{16.0\ \text{g}}{32.0\ \text{g mol}^{-1}} = 0.5\ \text{mol}

Number of molecules = moles × Avogadro’s number.

Number of molecules =

0.5 \times 6.02 \times 10^{23} = 3.01 \times 10^{23}

Answer:

\boxed{3.01 \times 10^{23}}

molecules

Correct option: C.

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