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- Let's say we have some hydrochloric acid, and a solution of sodium hydroxide. We know that hydrochloric acid is a strong acid so we can think about it as consisting of H+ and Cl-. Sodium hydroxide is a strong base, so in solution we're going to have sodium ions and hydroxide anions. Let's think about the products for this reaction. One product would be H+, and OH-. If you put H+ and OH- together, you form H20. So water is one of our products, and the other product would be what's leftover. We have Na+ and Cl-, so that gives us NaCl which is sodium chloride. This is an example of an acid base neutralization reaction where an acid reacts with the base to give you water and a salt. In this case, our salt is sodium chloride. Let's think about an aqueous solution of sodium chloride. You take some water, and you take some salt, and you dissolve your sodium chloride in water to make your solution. In solution, you're gonna have sodium cations and chloride anions. Let's think about what those would do with water. We know that the pH of water is seven. The pH of water is equal to seven. Sodium ions don't react with water so they're not going to affect the pH of our solution. You might think that the chloride anion could function as a weak base, and take a proton from water. However, that's not really gonna happen very well because the chloride anion, Cl-, this is the conjugate base to HCl. We know that HCl is a strong acid, and the stronger the acid, the weaker the conjugate base. With a very strong acid, we're gonna get a very weak conjugate base so the chloride anion can't really take protons from water very well so the pH is unaffected. The pH of our solution of sodium chloride is equal to seven. When you have a salt that was formed from a strong acid and a strong base, so sodium chloride was formed from a strong acid and a strong base, these salts form neutral solutions. So your pH should be equal to seven. Let's compare this situation to the salt that's formed from a weak acid, and a strong base. Over here we have acetic acid which we know is a weak acid. Then we have sodium hydroxide which is our strong base. In solution, we would have Na+ and OH-. Hydroxide is going to take the acidic proton on acetic acid, and this is the acidic proton on acetic acid. Once again, H+ and OH- give us H20. Let's think about the salt that forms here. If you take a proton away from acetic acid, you're left with acetate. You're left with CH3COO-, the acetate ion. We also still have sodium ions here. The sodium ion could ionically bond over here with your acetate anion. Here we formed sodium acetate. Let's think about an aqueous solution of sodium acetate. We know that the pH of water is seven. Let's think about what we have in solution. Once again , we have sodium cations which aren't going to react with water so that's not going to affect the pH at all. But the acetate anions will. Let's go ahead and write out what would happen, The acetate anions are going to react with water. Let's think about what would happen here. Let's put this at equilibrium. The acetate anions ... we know this is the conjugate base to acetic acid. This is gonna function as a Bronsted-Lowry base, and water is gonna function as a Bronsted-Lowry acid. Water is going to donate a proton to the acetate anion. If the acetate anion picks up a proton, we would form acetic acid, CH3COOH. So just add an H+ to the acetate anion. If the acetate anion takes a proton away from H20, if you take an H+ away from H20, you're left with OH- here. What has happened in our solution? The fact that the acetate anion is present has increased the concentration of hydroxide ions in soluiton. We've increased the concentration of hydroxide ions so the pH is no longer seven. We've increased the pH here. The pH should be greater than seven. An aqueous solution of sodium acetate is gonna have a pH of greater than seven. In general, this is true. If you have the salt, this salt here that was formed from a weak acid and a strong base, these salts form basic solutions with pHs greater than seven. Let's look at one more example. This is the example of a strong acid and a weak base. HCl is our strong acid, and ammonia is our weak base. So HCl is H+ and Cl-. We're going to donate a proton to NH3. If NH3 accepts a proton, it's gonna turn into NH4+ so this is ammonium. We also have some chloride anions. That would ionically interact with the ammonium so we form ammonium chloride here. So this is our salt. Let's think about an aqueous solution of ammonium chloride. Once again, for water the pH is seven, and the chloride anions aren't going to affect the pH. But the ammonium ion will. NH4+ will react with water and affect the pH. Let's look at the reactions. NH4+ + H20, so what's going to happen here. The ammonium ion is an acid. It's gonna function as a Bronsted-Lowry acid. It's going to donate a proton to water. Water's gonna function as a Bronsted-Lowry base, and accept a proton. If you take away an H+ from NH4+, you're left with NH3 or ammonia. If you add an H+ to H20, then you get H30+ or hydronium. What is happening in our solution? We've increased the concentration of hydronium ions. We've increased the concentration of hydronium, and we know that what that's gonna do to the pH. Our pH is no longer going to be seven. We're going to decrease the pH, so we have an acidic solution. The pH is now less than seven. An aqueous solution of ammonium chloride is going to have a pH of less than seven. A salt that's formed from a strong acid and a weak base is going to give you an acidic solution.