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Question Video: Identifying the Volatile Compound in a Reaction for Volatilization Gravimetry Chemistry

A student wants to determine the amount of sodium bicarbonate in a sample using volatilization gravimetry. The student reacts the sample with dilute sulfuric acid, resulting in the reaction 2 NaHCO₃ (aq) + H₂SO₄ (aq) ⟶ 2 CO₂ (g) + 2 H₂O (l) + Na₂SO₄ (aq). What volatile molecule will the student measure?

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Video Transcript

A student wants to determine the amount of sodium bicarbonate in a sample using volatilization gravimetry. The student reacts the sample with dilute sulfuric acid, resulting in the reaction 2 NaHCO3 (aqueous) plus H2SO4 (aqueous) reacts to produce 2 CO2 (gas) plus 2 H2O (liquid) plus Na2SO4 (aqueous). What volatile molecule will the student measure?

To answer this question, we need to understand what volatilization gravimetry is and what a volatile molecule is. Volatilization gravimetry is a mass analysis method that uses thermal or chemical energy to separate substances in order to measure their masses. And a volatile compound is a compound with a low boiling point. Volatile molecules include carbon dioxide, chlorine gas, or nitrogen gas. Volatile molecules that are formed in a chemical reaction will be liberated from the reaction mixture, or we can heat the reaction mixture if necessary in order to liberate them.

In volatilization gravimetry, we can either measure the amount of volatile molecule that is liberated, or measure the mass of the final product to determine how much of the volatile molecule was lost. If the reactants or products are solid, then they are easy to weigh and measure. However, if we look at the chemical equation given in the question, we can see that our reactants are both in the aqueous state. This makes measuring their mass very difficult. Now, let us take a look at the reaction we are given to determine the volatile molecule the student will measure.

The reactants NaHCO3 and H2SO4 are in an aqueous solution. The chemical species are dissolved in water making it difficult to measure the mass as we would for a solid. Now, let us look at the products. As the ions react in the solution, CO2, H2O, and Na2SO4 form. Since Na2SO4 would dissolve in the water since it is aqueous, this would make it difficult to measure. The water is the solvent in our reaction, dissolving the ions. So, it would not be ideal to measure it since it is not pure water. You could distill the water and condense it to obtain pure water, but this would be an extra step and more time-consuming.

Now, we are left with CO2. We do not need to heat the reaction mixture for the CO2 to escape since it is already a gas. We could simply collect any gas produced as a result of the reaction. Since the CO2 can be collected, we can determine the mass or volume of CO2. Then, we can calculate the number of moles produced of CO2. Using this in the chemical equation, we can identify that for every two moles of CO2 produced, there are two moles of NaHCO3 needed by looking at the coefficients. This mole ratio allows us to calculate the mass of NaHCO3 in the sample.

So, in this experiment, the volatile molecule the student will measure is carbon dioxide.

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