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How many grams of NaCN would you need to dissolve in enough water to make exactly $250 \mathrm{mL}$ of solution with a pH of $10.00 ?$

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$$7 \cdot 4 \times 10^{-3} g$$

Chemistry 102

Chapter 15

Acids and Bases

Liquids

Carleton College

University of Central Florida

Rice University

Lectures

03:07

A liquid is a nearly incompressible fluid that conforms to the shape of its container but retains a (nearly) constant volume independent of pressure. As such, a liquid is one of the four fundamental states of matter (the others being solid, gas and plasma). A liquid is made up of tiny vibrating particles of matter, such as atoms, held together by intermolecular bonds. Water is, by far, the most common liquid on Earth. Like a gas, a liquid is able to flow and take the shape of a container. Most liquids resist compression, although others can be compressed. Unlike a gas, a liquid does not disperse to fill every space of a container, and maintains a fairly constant density. A distinctive property of the liquid state is surface tension, leading to wetting phenomena.

04:38

A liquid is a state of matter in which a substance changes its shape easily and takes the form of its container, and in which the substance retains a constant volume independent of pressure. As a result of this, a liquid does not maintain a definite shape, and its volume is variable. The characteristic properties of a liquid are surface tension, viscosity, and capillarity. The liquid state has a definite volume, but it also has a definite surface. The volume is uniform throughout the whole of the liquid. Solids have a fixed shape and a definite volume, but they do not have a definite surface. The volume of a solid does not vary, but the volume of a liquid may vary.

02:58

A solution of sodium cyani…

02:00

02:56

A solution of NaOH has a p…

01:23

How much NaOH (in grams) i…

02:37

How much $\mathrm{NaOH}$ (…

Hello. And let's do another pretty challenging problem. And this problem is going to look at relationships for ph. Solution, solution. Prep salts, etcetera, etcetera. Okay, we're has to solve for how many grams of sodium cyanide would we need to dissolve in water if we wanted to make 250 milliliters, which, of course, is 0.250 and get rid of that zero. Because that's not significant. I don't think No. 2 50 leaders of a solution that has a ph of 10.0 Okay, so this has a ph of 10.0 and we're not really dealing with an an acid here, So we were gonna have to be looking up what our original concentrations are and figuring out all of our initials and changes. Um, we're gonna write out our chemical equation first and our chemical equation. I think I'll just go ahead and go to a new page. So I have plenty room Is CNN cyanide. Molly hurts. I tonight. I on. Of course, water is always involved in these Equus things, and my products are going to be h c N and ohh minus. Ok, there we go, and we're going to do an ice table, and this is going to be our X. Don't consider water, and we know that these air zero we know that these are going to be plus x, x and X. I guess they're not gonna be. Excuse me. I've got my pH here. So I know that, um, the pH I'll write this down here might after rate reraise this the p h equals 10.0 And we're looking at a base here. So we also know that our p o age for this has to be 4.0 are Ph was 4.0 I don't even need a calculator to do this one. Because if I take the negative log of this, I will get one point zero times 10 to the minus fourth. That is my concentration for my eight. Ohh, Right here, one. I better do Sig figs. We don't get to mixed up 1.0 times. 10 to the minus fourth. This will also be 1.0 times 10 to the minus forth. Those air polarities. I don't really need to fill this one out right now, but I will. There's my fourth. This is going to decreasing concentration by the amount. So it'll be X minus 1.0 times 10 to the minus fourth, and we can write our K expression. Um, I said I was going to erase this, and I am going to erase this now. Always back up if you need to go look at that. So my k a r k b expression for this one will be a CNN and, oh, age and my numerator. And in the denominator, it will be the c N minus. We could look up K B and K B is given in this baby equals 2.0 times 10 to the minus five. So substituting we have 2.0 times 10 to the minus five equals. And then I'm gonna take one again. I'm just putting my one there 1.0 times. 10 to the minus fourth, that quantity squared, and then we're going to take this over X minus 1.0 times 10 to the minus fourth. And I'm lazy on this. So I'm going to say that this is going to be equivalent to X, so we sell for X here because this is a pretty low number. When we solve for X, we're going to end up with 6.0 times 10 to the minus fourth Moller. And that's for the CNN Moller CN minus. Okay, Almost done. Let me write that number down again. We have 6.0 times. 10 to the minus fourth Moeller, CNN and we were asked to find how many grams of n a C N of sodium cyanide would be needed. For How many? Grams 40.250 leaders. Okay, Pretty simple. Here. Um, we're going to convert. I'm gonna take my 2 50 which already have, um I'm gonna take six 0.0 times 10 to the minus fourth Moles per leader. Times 0.250 leaders and then the molar mass for in a CNN is 49 0.1 grams per mole. And the answer is 7.4 times 10 to the negative. Three grands of sodium cyanide. You? That was a toughie.

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