Animated Solution for Chemistry - s and p-Block Elements: 5 g of zinc is treated separately with an excess of (1) dilute hydrochloric acid and (2) aqueous sodium hydroxide. The ratio of the volume of $H_2$ evolved in these two reactions is

Imagine you are standing in a chemistry laboratory. On your workbench, you have two identical test tubes. In each of them, you carefully drop exactly $5\text{ g}$ of zinc metal. Now, here is where things get interesting. In the first test tube, you pour dilute hydrochloric acid ($\text{HCl}$). In the second, you pour aqueous sodium hydroxide ($\text{NaOH}$).

Zinc is a fascinating element because it is amphoteric. This means it has the unique ability to react with both strong acids and strong bases. Let's dive into the chemical equations to see exactly what happens in each case.

Let's focus on the first test tube with the acid. When zinc comes into contact with hydrochloric acid, a vigorous single displacement reaction occurs. The zinc atoms push the hydrogen out of the acid, forming aqueous zinc chloride, and releasing hydrogen gas as bubbles.

The balanced chemical equation for this reaction is:

If we look at the stoichiometry of this equation, it is crystal clear. Exactly $1\text{ mole}$ of solid zinc reacts to produce exactly $1\text{ mole}$ of hydrogen gas. Keep this $1:1$ ratio in mind as we move to the next step.

Now, let's shift our attention to the second test tube containing the strong base, sodium hydroxide. You might intuitively think a metal wouldn't react with a base, but remember, zinc is amphoteric! It reacts to form a soluble complex salt known as sodium zincate, and just like before, it liberates hydrogen gas.

The balanced equation for this reaction is:

Notice the coefficients? Once again, exactly $1\text{ mole}$ of zinc yields exactly $1\text{ mole}$ of hydrogen gas. The chemistry might be different, and the salt formed is different, but the gas output ratio per mole of zinc is identical.

So, we have established the theoretical mole ratios. But what about our specific experiment? The problem states we started with exactly $5\text{ g}$ of zinc in both test tubes.

Since the mass is identical, the number of moles of zinc reacting in both tubes is also perfectly identical ($n=\frac{5}{65.4}\text{ moles}$). And because both reactions produce one mole of hydrogen for every mole of zinc consumed, the total moles of hydrogen gas bubbling out of both test tubes will be exactly the same.

We are almost at the finish line. We know the moles of hydrogen gas are equal. But the question asks for the ratio of their volumes. This is where Avogadro's Law comes to the rescue. It states that at a constant temperature and pressure, the volume of a gas is directly proportional to its number of moles ($V\propto n$).

Since the moles are equal, the volumes must be equal too! Therefore, the ratio of the volume of hydrogen evolved in the two reactions is simply $1:1$.