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WAEC Chemistry 2025 Paper

Question : 1

Total: 50

Which of the following pairs of gases cannot be collected by downward delivery?

Solution:

Concept:

Downward delivery is used only for gases that are denser than air.

Explanation:

Air has an average molar mass of about

29\ \mathrm{g\,mol^{-1}}

A gas can be collected by downward delivery only if its molar mass is greater than

29\ \mathrm{g\,mol^{-1}}

Both

\mathrm{CO}

and

\mathrm{N_2}

have a molar mass of

28\ \mathrm{g\,mol^{-1}}

So they are slightly lighter than air and would rise upward instead of settling downward.

Hence,

\mathrm{CO}

and

\mathrm{N_2}

cannot be collected by downward delivery.

They should be collected by upward delivery instead.

Answer:

\mathrm{CO}

and

\mathrm{N_2}

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