Question Video: Identifying the Weakest Base in a Set from Their 𝐾_b Values | Nagwa Question Video: Identifying the Weakest Base in a Set from Their 𝐾_b Values | Nagwa

Question Video: Identifying the Weakest Base in a Set from Their 𝐾_b Values Chemistry • Third Year of Secondary School

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The ability of a base to accept protons could be deduced from its base dissociation constant (𝐾_b). The base dissociation constant of some bases is listed in the table. Which of the bases listed in the table is the weakest?

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Video Transcript

The ability of a base to accept protons could be deduced from its base dissociation constant 𝐾 𝑏. The base dissociation constant of some bases is listed in the table below. Which of the bases listed in the table is the weakest?

It is useful for chemists to be able to quantify the strength of bases. We can do this using the base dissociation constants. The base dissociation constant, abbreviated as 𝐾 𝑏, is the equilibrium constant for the reaction of a base with water. The general equation for this reaction can be written as base, B, plus water. The base and water react reversibly to produce the conjugate acid, BH+, and hydroxide ions.

We can write the base dissociation constant for this reaction as the concentration of the products multiplied together and divided by the concentration of the reactants multiplied together. We do not write water into the expression as it is a liquid and not an aqueous solution with a concentration.

A weak base does not completely dissociate in water. So, the equilibrium lies to the left. This means that at equilibrium, the concentration of the ions will be low, while the concentration of the base is high. This results in a small 𝐾 𝑏 value. The weaker the base, the farther the equilibrium lies to the left. The lower the concentration of the ions at equilibrium, the lower the 𝐾 𝑏 value. The weakest base from the bases listed in the table will have the smallest 𝐾 𝑏 value.

As we look at the values in the table, it is worth mentioning that while no units are listed, 𝐾 𝑏 values are measured by the unit moles per liter. The smallest 𝐾 𝑏 value listed in the table is 4.3 times 10 to the negative 10, which corresponds to base E. Therefore, of the bases listed in the table, the weakest is base E.

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