Video: Identifying the Gas That Gives a Weakly Acidic Solution in a Set of Chemical Formulas

Which of the following corresponds to a gas that gives a weakly acidic solution when added to water? [A] SO₂ [B] KBr [C] CF₄ [D] HCl [E] C₆H₆


Video Transcript

Which of the following corresponds to a gas that gives a weakly acidic solution when added to water? A) SO₂. B) KBr. C) CF₄. D) HCl. E) C₆H₆.

We’re being asked about a substance which is a gas under normal circumstances. Gases are substances where the particles are separated by some considerable distance and those particles are in chaotic random motion. When we identify the physical state of a substance, we normally assume that we’re talking about room temperature and atmospheric pressure. So, let’s go ahead and try to identify the gases in our list of substances in this question.

SO₂ is sulphur dioxide. Sulphur dioxide at room temperature and pressure is a toxic gas. It’s often associated with the combustion of coal and heavy fuel oil that contains sulphur as impurities. KBr is potassium bromide. Although potassium bromide is very soluble in water, at room temperature and pressure it’s an ionic solid. It has a very high melting point. It’s not a gas. CF₄ is carbon tetrafluoride. CF₄ has a very low boiling point of minus 128 degrees centigrade. At room temperature and pressure, it is a gas. It’s used in refrigeration systems. And it’s also a potent greenhouse gas as well.

HCl is hydrogen chloride. HCl is a colorless gas at room temperature and pressure. Although, it does form white fumes when it comes into contact with atmospheric water vapor. So, HCl is a gas. C₆H₆ is known as benzene. With a boiling point of 80 degrees centigrade, benzene is a liquid at room temperature and pressure. It’s also a known carcinogen, so you won’t encounter it in schools and colleges. Now that we’ve identified substances A, Sulphur dioxide; C, carbon tetrafluoride; and D, hydrogen chloride as gases at room temperature and pressure, we can eliminate B and E as incorrect answers.

Next, we need to consider which of our gases A, C, or D are in fact soluble in water and in fact give an acidic solution when they dissolve. In particular, the solution must be weakly acidic. Remember that a weak acid is an acid that is only partly ionized in aqueous solution. If we look very closely at answer C, we have CF₄, which is a nonpolar molecule. There is no overall dipole within this molecule. Therefore, it cannot dissolve in water, and it cannot form an acidic solution. C is not a correct answer.

Sulphur dioxide is highly soluble in water. As a nonmetal oxide, it forms an acidic solution. Sulfurous acid, or sulfuric four acid as it’s otherwise known, is formed in this case. The sulfurous acid, however, is only partly ionized in water. The equilibrium shown here lies far to the left-hand side. Not many hydrogen ions are released by the sulfurous acid in water, and the solution is only weakly acidic. The pH of this solution would be approximately three to four.

We have found our gaseous substance that forms a weakly acidic solution. And answer A is looking promising. But let’s first check what happens when hydrogen chloride gases dissolved in water? HCl is also highly soluble in water. But when HCl dissolves in water, almost every single molecule ionizes to release hydrogen ions. This creates a very strongly acidic solution. So, we would consider hydrogen chloride to be a soluble gas, but it would produce a strongly acidic solution when added to water. Therefore, D is not a correct answer. The correct answer is sulphur dioxide, which is answer A.

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