Weak Acid Strong Base Titrations 2

by Jules Bruno
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now in order to create a buffer. 7.510 g of sodium cyanide is mixed with 100 ml of 1000.250 Mueller Hydro sonic acid. What is the pH of the buffer solution after the addition of 175 MLS off 1750.300? Mueller sodium hydride? All right, so hydro Santic acid represents a week binary acid. It has a name similar to sodium cyanide. This must represent the conjugate base. Now remember, the conjugate base has one less hydrogen than the acid form that hydrogen that it has, one less of has been replaced by a metal. So if hydro sonic acid is H. C N. Sodium cyanide is n a CNN, then we're gonna stay here. That sodium hydride here represents a strong base. That's because sodium Group won a medal connected to the hydride ion. Remember, if a group win a medal is connected to H minus O H minus and H two minus or O to minus, it will form a strong base. So we have a strong based species with two weeks species. Weak acid in contact base are both weak. Any time we have strong and weak species reacting. We need to set up an ice CF chart. Remember in the I C F chart, whatever its strong has to be a reactant. So the strong base has to be reacted because it is a base. It's going to react with its opposite. Its opposite is an asset, so Xcn would react with it. Now remember, what do acids do? Acids giveaway H plus So wolf the H puzzle connect with the H minus two. Give us hte too. Then the A C N will be formed in place as the conjugate base for one of the products. This year represents our chemical equation between the interaction amongst the strong base and the weak acid. Remember, since we have weak and strong, we have a nice CF chart in a nice CF chart. We use moles. First, we're going to realize that they gave us grams of our conjugate base. So we're gonna have to change that into moles. We have the formula for the conjugate base so we can figure out what the molecular weight of it ISS it has in it. One sodium, one carbon and one nitrogen. Looking up their masses on the periodic table. Sodium weighs 22.99 g. Carbon is 12.1 g and nitrogenous 14. g. When we write all those masses down, that gives us a combined mass of 49.1 g. So we're gonna take that 7.510 g of sodium cyanide divided by its molecular mass. And we'll know the moles of our conjugal base are most of concrete. Base comes out 2.1532 moles. So that's our beginning. Moles of our conjugate base Remember, uh, it's not always going to be that our products are zero initially. In this case, we actually have a beginning amount for our conjugal face. Now we're going to have to divide our milliliters by 1000 to get leaders and then multiply them by their mole Aridjis. Because remember, moles equals leaders times molar ity. So when we divide 100 miles by 1000 to get leaders, we multiplied by the 10000.250 Moeller That's gonna give me 0.25 moles of the weak acid divide 175 MLS by 1000 multiplied by its molar ity. And that gives me 0.525 moles next, remember? Look it on Lee the react inside the smaller moles will subtract from the larger moles. So minus 0.25 minus 0. This comes out of zero here. We're gonna still have some of our strong base remaining. Whatever happens on the react inside, the opposite happens on the product side because of the conservation of mass. So bring down everything at the end. What do we have left at the end? We have strong base remaining, and we have conjugal based remaining. We no longer have a buffer because the weak acid is completely gone. Remember, we have strong species left, so that means we're after the equivalence point. Also remember, if we have strong base and conjugate base, the strong base will have a bigger impact on the overall pH. So focus on the strong species. We're gonna figure out what its new concentration is, so we need to take its moles okay and divided by our total volume. What's our total volume? Well, we use the 100 miles of the weak acid and 175 miles of the strong base. Change those into leaders and add them together. And that gives us our total volume on the bottom. So that's gonna give me 0.100 Mueller, sodium hydride. Because we have a strong base, we confined P O h, which is negative log of this case off the strong base. So that's gonna give me negative log off 0.100 which comes out toe one. But remember, we don't want p o h. We want Ph. So here we say pH equals 14 minus ph. So that's 14 minus one. So our Ph you would be 13. So remember, if we have ah, a week species, any weak species reacting with strong species, we have to set up a nice CF chart. Whatever strong has to be reacted in this case is a strong base. It will react with its opposite. Its chemical opposite is an acid, so it has to react with a weak acid. Use that to help set up the initial equation. Once you do that, you can set up your I C f chart making sure the units are in moles. Remember, you look at the react inside the smaller moles subtract from the larger moles that will help you to determine what your final amounts of everything will be. In this case, we no longer have a buffer, but we have a strong species remaining. From that information. We can take the strong species and figure out its concentration. From there, you can figure out P O. H because it is a strong base. And once we know P. O. H, we confined pH. Remember these steps that are necessary for answering questions that deal with a weak acid and strong base after the equivalents point?