Problem 22 - Chemistry and Chemical Reactivity

Question

Which of the following common food additives would give a basic solution when dissolved in water? (a) \(\mathrm{NaNO}_{3}\) (used as a meat preservative) (b) \(\mathrm{NaC}_{6} \mathrm{H}_{5} \mathrm{CO}_{2}\) (sodium benzoate; used as a soft-drink preservative) (c) \(\mathrm{Na}_{2} \mathrm{HPO}_{4}\) (used as an emulsifier in the manufacture of pasteurized cheese)

Step-by-Step Solution

Answer

(b) Sodium benzoate and (c) sodium hydrogen phosphate give basic solutions.

Step 1: Understand the Nature of the Salts

Each of the given compounds is a salt, and the nature of the solution when each salt dissolves in water depends on the nature of the ions produced. We need to consider whether the anion (negative ion) of the salt weakly reacts with water to produce hydroxide ions, which make the solution basic.

Step 2: Analyze Sodium Nitrate ( \(\mathrm{NaNO}_{3}\) )

When dissolved in water, sodium nitrate separates into Na⁺ and NO₃⁻ ions. Nitrate ion (NO₃⁻) is the conjugate base of a strong acid, nitric acid (HNO₃) and does not react with water to any significant extent, so the solution remains neutral.

Step 3: Analyze Sodium Benzoate ( \(\mathrm{NaC}_{6} \mathrm{H}_{5} \mathrm{CO}_{2}\) )

Sodium benzoate dissolves in water to form Na⁺ and C₆H₅COO⁻ ions. The benzoate ion (C₆H₅COO⁻), being the conjugate base of benzoic acid, a weak acid, will react with water to produce hydroxide ions (OH⁻) and the solution will become basic.

Step 4: Analyze Sodium Hydrogen Phosphate ( \(\mathrm{Na}_{2} \mathrm{HPO}_{4}\) )

Sodium hydrogen phosphate dissolves to form Na⁺ and HPO₄²⁻ ions. The HPO₄²⁻ ion is the conjugate base of a weak acid (H₂PO₄⁻), and it can react with water to form OH⁻ ions, making the solution basic.

Step 5: Conclusion

After analyzing all the options, both sodium benzoate and sodium hydrogen phosphate are expected to produce basic solutions when dissolved in water due to their anions reacting with water.

Key Concepts

Sodium Benzoate

Sodium benzoate is a common preservative found in many food and beverage products, specifically soft drinks. When sodium benzoate (\(\mathrm{NaC}_{6} \mathrm{H}_{5} \mathrm{CO}_{2}\)) dissolves in water, it breaks apart into sodium ions (\(\text{Na}^+\)) and benzoate ions (\(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{COO}^-\)).
The benzoate ion is important because it acts as a conjugate base.
The term "conjugate base" refers to what remains after an acid donates a proton.
In this case, benzoate is the conjugate base of benzoic acid, which is a weak acid. Since benzoic acid does not completely donate all its protons to water, its conjugate base (benzoate) reverses this by gaining a proton quickly.
When the benzoate ion interacts with water, it has a tendency to accept a proton.
This leads to the formation of hydroxide ions (\(\text{OH}^-\)), which increases the basicity of the solution. Therefore, when sodium benzoate is dissolved in water, it leads to a basic solution.

Sodium Hydrogen Phosphate

Sodium hydrogen phosphate is used widely in food processing, particularly in cheese manufacturing as an emulsifier. When sodium hydrogen phosphate (\(\mathrm{Na}_{2} \mathrm{HPO}_{4}\)) dissolves in water, it splits into sodium ions (\(\text{Na}^+\)) and hydrogen phosphate ions (\(\mathrm{HPO}_4^{2-}\)).
The hydrogen phosphate ion is another example of a conjugate base. It is the conjugate base of dihydrogen phosphate (\(\mathrm{H}_{2} \mathrm{PO}_{4}^-\)), which is a weak acid.
Just like the benzoate ion, the hydrogen phosphate ion can react with water. During this interaction, it accepts a proton from a water molecule, leaving behind a hydroxide ion (\(\text{OH}^-\)).
This presence of extra hydroxide ions is what makes the solution basic.
In summary, when sodium hydrogen phosphate dissolves, it converts some water molecules into hydroxide ions, and this is what contributes to the basic nature of the solution.

Conjugate Base

Understanding conjugate bases is essential to determine the basicity of solutions. When an acid donates a proton (\(\text{H}^+\)), what is left is called its conjugate base. This means a conjugate base forms when a proton is removed from an acid.
For instance, as seen in benzoate and hydrogen phosphate ions, conjugate bases play a crucial role in determining the nature of a solution when these ions dissolve in water.
Conjugate bases are generally the species that have the capability to accept protons, thereby generating hydroxide ions (\(\text{OH}^-\)) in water, increasing the solution's pH and making it basic.

• For benzoate, the conjugate base of a weak acid (benzoic acid) leads to a basic solution.
• For hydrogen phosphate, also the conjugate base of a weak acid (dihydrogen phosphate), the same occurs.

A conjugate base is not always strong enough to make a solution basic; the original acid must be weak. In solutions where a strong acid has lost a proton, the resulting conjugate base has minimal impact on pH, as it does not tend to take back protons.Thus, the basicity of a solution containing salts like sodium benzoate or sodium hydrogen phosphate is largely attributable to the strong tendency of their corresponding conjugate bases to accept protons and form hydroxide ions.