Video: Identifying the Solution with at pH between 7 and 13 at 298 K in a Set of Single-Component Solutions of Known Concentration

Which of the following solutions has a pH greater than 7 but less than 13 at 298 K? [A] Solution A [B] Solution B [C] Solution C [D] Solution D [E] Solution E

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Video Transcript

Which of the following solutions has a pH greater than seven but less than 13 at 298 kelvin?

In this question, we’re being asked to consider the pH of three separate solutions. That is, a monoprotic strong acid, called hydrobromic acid, a strong base called lithium hydroxide, and a monoprotic weak acid, called methanoic acid. Additionally, we need to consider the pH of two solutions that result in mixtures of these acids and bases.

Firstly, let us consider hydrobromic acid. It reacts with water molecules to form hydronium ions and bromide ions. As hydrobromic acid is a strong acid, this ionization process is almost complete in water. You may see the reaction written as HBr turned into hydrogen ions and bromide ions.

In a 0.2-molar solution of hydrobromic acid, we would expect the hydrogen ion concentration to be 0.2 molar also. As the pH of a solution is defined as the negative logarithm to the base of 10 of the hydrogen ion concentration. Then the pH of this solution would be negative logarithm to base 10 of 0.2.

Without a calculator to hand, given that this is a strong acid, we can be sure that the pH of this solution will be less than one. This is not within the range specified in the question. So therefore, our hydrobromic acid solution is not the correct answer.

Methanoic acid is a monoprotic weak acid. It also reacts with water molecules to release hydronium ions and methanoate ions. However, this process is a reversible reaction. And methanoic acid is only partly ionized in water. The concentration of methanoic acid molecules is much, much greater than the concentration of hydronium ions and methanoate ions in this solution.

We would need the Ka value of this methanoic acid to calculate the precise pH of this solution. However, as it’s a weak acid, we can rest assured the pH will be about four. This is not within the range specified by the question. So our methanoic acid solution is not the correct answer either.

Lithium hydroxide is a group one metal hydroxide. And like all the group one metal hydroxides, it’s highly soluble in water. It dissolves to release lithium ions and hydroxide ions. In this solution, the concentration of hydroxide ions will far exceed the concentration of hydrogen ions. We are dealing with a strongly basic solution here.

We would need the Ka value of water at 298 kelvin to calculate the precise pH of this strongly basic solution. But we can be sure that its pH is above 13. This lithium hydroxide solution is not the correct answer either.

In the remaining two solutions, we are mixing acids and bases together to form salts and water. In the reaction of methanoic acid with lithium hydroxide, it is the less familiar salts lithium methanoate that is formed. In each case, the reactions are one-to-one mole ratios. And if we mix equal volumes of equimolar solutions, it would appear that we have perfect neutralizations taking place in both cases.

When we mix equal volumes of equimolar solutions of hydrobromic acid and lithium hydroxide, the neutral salt lithium bromide is formed. This salt has no acidic or basic character. And the solution will be exactly neutral. We would expect the pH to be precisely seven. This lies below the range specified in the question. So this lithium bromide solution is not the correct answer either.

In the case of the lithium methanoate salt solution, we have the conjugate base of a weak acid. The methanoate ions can accept protons from solvent water molecules. This will release hydroxide ions into the solution. The consequence of this is that the pH of our lithium methanoate salt solution will be slightly above seven. This is within the range of the pH specified in the question. The lithium methanoate salt solution is therefore the correct answer.

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