Video: Q13paper3_2

Which of the following solutions has a pH greater than 2 but less than 7 at 298 K? [A] Option A [B] Option B [C] Option C [D] Option D [E] Option E

04:31

Video Transcript

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

In this question, we’re being asked to consider the pH of three individual solutions, 0.2 mole per liter hydrobromic acid, 0.2 mole per liter lithium hydroxide, and 0.2 mole per liter methanoic acid. In addition, we’re being asked to consider the pH of the resulting solution when two pairs of substances are mixed together. Let us consider the pH of the individual solutions first to establish if they are likely answers.

Hydrobromic acid reacts with water molecules to form hydronium ions. As it’s a strong acid, this process is almost complete in water. The concentration of hydrogen ions in a 0.2-mole-per-liter solution of hydrobromic acid is 0.2 mole per liter. The pH of this solution would be less than one. The pH of a strong-acid solution is defined as the minus log to base 10 of the hydrogen ion concentration. If you had a calculator available and you found the minus log to base 10 of 0.2, the pH of this solution is, in fact, 0.7. As we’re being asked to find the pH of a solution which is greater than two but less than seven, this hydrobromic acid solution is not the correct answer here.

Lithium hydroxide is a strong base. Like all of the group one metal hydroxides, it’s very soluble in water. Many hydroxide ions are released into the solution, and the concentration of hydroxide ions is far greater than the concentration of hydrogen ions. We would expect the solution to have a pH in excess of 12. The pH of this lithium hydroxide solution does not meet the criteria in the question, so it’s not the correct answer here.

In contrast to hydrobromic acid, methanoic acid is a monoprotic weak acid. It reacts with water molecules to produce hydronium ions and methanoate ions. However, the ionization in water is far from complete. As methanoic acid is a weak acid, we would need its 𝐾 𝑎 value to find the precise hydrogen ion concentration of this solution. However, we can be sure that its pH will be in the region of pH four. The pH of this methanoic acid solution could certainly lie between pH two and pH seven. It could be the correct answer here.

When we mix equal volumes of equimolar solutions of hydrobromic acid and lithium hydroxide, an acid–base reaction is taking place. The neutral salt lithium bromide is formed, which has no acidic or basic character. Equal moles of hydrobromic acid and lithium hydroxide are involved in this reaction, and the resulting solution would be perfectly neutral. We would expect its pH to be seven. As we are looking for a solution that has a pH of less than seven, this lithium bromide salt solution is not the correct answer.

In the reaction of methanoic acid with lithium hydroxide, the salt lithium methanoate is formed. The reaction is a one-to-one-mole ratio. And on the face of it, when we mix equal volumes of equimolar solutions of methanoic acid and lithium hydroxide, it would appear that we have a perfect neutralization reaction taking place. Although we could expect the pH of the resulting solution to be precisely seven, the methanoate ions produced are in fact the conjugate base for weak acid. They have basic character. They can accept protons from solvent water molecules releasing hydroxide ions into the solution. The pH of this lithium methanoate solution will be slightly above 7.0. It is therefore not the correct answer either.

Therefore, the only solution having a pH greater than two but less than seven is the methanoic acid solution.

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