Hi, this is Mrs. Hollick and this is your podcast on molarity. So again, we're talking about quantifying concentrations of solutions. So this unit is molarity.
Again, look at here's a picture of different solutions. And this time, this graphic actually shows you little particles that are in the solution. Notice how here's a dilute solution. You can see three or four particles in there. And we're getting more concentrated.
Notice it's getting darker and it's getting more concentrated. So this one. has a higher molarity than this one, if we're going to quantify it.
Let's talk about what molarity actually is. Molarity is the moles of solute divided by the liter of solution. So it's similar to the mass volume, but in this case we're using moles per liter. We're using a volume. Now the volume always has to be liters, and the unit up here always has to be moles.
So molarity you want to memorize is moles per liter. You want to memorize this formula. We're going to use this.
This is the most common concentration unit that you will use in chemistry. So, for example, a 2-molar sodium chloride solution would read a 2-molar sodium chloride solution. 2 capital M would be read as molar. What that means is that there's 2 moles.
sodium chloride dissolved in one liter of solution. Let's take a look at some calculations so we can start quantifying molarity. It says what is the molarity of a two liter solution that contains 0.4 moles of NaCl? So again you want to memorize this equation.
Molarity is equal to moles over liter and it's always solution on the bottom if you've noticed so far. With percent by mass and mass volume now we're on molarity. There's always it's always solution on the bottom What's the molarity would be x moles and we need liters moles are moles it says we have point four zero zero moles and Our solution is two point zero zero liters So what you can do when you're first getting started is you can set it up by writing it out on the side Then what you can do is plug it into the equation so x equals 0.400 moles over 2.00 liters.
And then when you divide those, you will get 0.200 molar. Now you can write capital M for molar, or you could write moles per liter. It's up to you. Either one is equivalent.
Sig figs come from the least amount in the two numbers that you're dividing. Three sig figs here, three sig figs here, three sig figs in my answer. It says how many grams of salt are actually in this solution?
Which unit is in the molarity equation that would be connected to grams? You should be saying moles. Well, they told us in the solution we had 0.4 moles of sodium chloride.
How many grams is that? Well, you should be thinking back to our unit on converting moles into grams. And that's all that this is, is a simple calculation.
We have 0.4 moles of NaCl. How do we change that to grams? Multiply by the formula mass over 1 mole. That's it. So that will come out to 23.4 grams.
Three significant figures here, and we want three significant figures in our answer. The 58.44 is literally just sodium 22.99 plus chlorine 35.45. The atomic mass is added up to create their formula mass. Let's look at another example.
It says if 56.7 grams of potassium nitrate are dissolved in water to make 3.50 liters of solution, what is the molarity? All right, so again, molarity is moles over liters. What is the molarity?
Would be our x, right? Do we have moles? No. Do we have something that can get us to moles? Absolutely.
We have a mass. Do we have our liters? 3.50 liters, yes we do.
So we know that 3.50 liters will go here. So now we have to get moles from our grams. So let's come down here, 56.7 grams of...
potassium nitrate change that to moles just like we've been in the past so one mole of kn03 has a mass of 101.11 grams. Where did I get that from? The mass of potassium, the mass of nitrogen, and the mass of 3-oxygen added up. That's the formula mass. So then when we calculate that, we will get 0.561 moles of KNO3.
Well now we have our moles of solute, and we have our volume of solution. We can calculate the molarity. So 0.561.
Moles will go on top, divided by 3.50 liters, and that will be equal to 0.160 molar. Or you could say moles per liter, either one. Significant figures, notice I had 3 here, so I carried 3 to this answer, which was carried up to here.
Now I have 3 compared to 3, so I want 3 in my answer. It's always the least amount in what you're dividing. Example 3. How many grams of potassium nitrate are needed to make 1.50 liters of a 0.5 molar solution?
Okay, we have something different here. So again, molarity is moles over liter. What do we have? We have 1.50 liters of a solution.
We have this. We have a 0.5 molar solution. We have this.
So we have this and we have this. We don't have moles. Can moles get us to the question, grams?
Sure. So first we're going to solve for moles, then we're going to convert those moles into grams. So, 0.500 molar is equal to x over 1.50 liters.
Now how do I solve that? Well if you think about it like fractions in math, we could put this over 1, and then you cross multiply. So x times 1 would be x, and then 0.5 times 1.5 would be 0.750 moles. Now we need to know the substance.
It's KNO3, so that we convert it to grams. Now this is how many moles we have. All we want to do is change the moles into grams by multiplying by the formula mass, because moles will go down here.
And we know 1 mole of KNO3, we can figure out the mass of that, and it's just 101.11 grams. When you multiply 0.750 times 101.11, you will get 75.8 grams. And that's the answer to the question, how many grams of potassium nitrate are needed to make 1.5 liters of a 0.5 molar solution.
Last example. What is the molarity of a solution that has a volume of 450 milliliters and contains 12.45 grams of sodium hydroxide? Do we have all of the pieces of our equation that we need? What is the molarity, so that will be x.
Do we have moles? No. We have grams. Can grams get us to moles?
Yes, they can. Do we have liters? No, we have milliliters.
Can we get milliliters into liters? Sure. So let's take one conversion at a time.
12.45 grams sodium hydroxide. Sodium is Na plus 1. Hydroxide, OH minus 1, crisscross. NaOH, let's convert that into moles.
1 mole of NaOH divided by the formula mass, 40.00 grams. That's the formula mass when you add up sodium. One oxygen and one hydrogen. Okay, that will give you 0.3113 moles of NaOH.
Now, our milliliters, 450 milliliters. Well, again, there's 1,000 milliliters in one liter. How do we change them? We're going to take the decimal point and move it back three places.
So that will be equal to 0. 0.4500 liters. So now we have moles, now we have liters, now we can calculate molarity. So the moles divided by the liters.
Why did I keep these zeros here? How many significant figures is our original volume? Four.
So when you convert it, you want to make sure you conserve those significant figures. That's why I did that. This answer is 0.6918 molar. Four significant figures, four significant figures, four significant figures.
So if you have any questions regarding molarity, I kind of went through every different type that you will see. There's lots of practice in your packet. Write those questions down, and when you get to class, you want to ask your teacher. and then you can start your practice.
Thanks for your attention!