**Molality (m) **represents the amount of solute dissolved per kilogram of solvent.

Understanding Molality

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example

## Molality

2m

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morality is depicted as moles of solute per kilograms of solvent morality, which is lower case m. Here we have moles of our salute which is a smaller part of our solution, divided by kilograms of the solvent, the larger portion of our solution. Now this is similar to Mole Arat e, which has capital m. It too, has moles of solute as the numerator. But the denominator is actually leaders of solution. Because of their similarities. There are questions which at times we'll ask you to interchange between the two. Now, in the same way we can expand polarity. The same approach can be applied to morality. So let's say that we have 0.30 mobile of sodium chloride. What does this translate into? Well, this just means that we have 0.30 moles of the Salyut, which is sodium chloride divided by 1 kg of our solvent, which is usually water. Let's say, for example, that we had to five motile off glucose in an acquis solution. Now again, 0.25 model. The number itself means that's how many moles we have off the Salyut. So be 0.25 moles of glucose here we say aqueous solution. Acquis means that are solvent is water, so that would be 1 kg off water. So the take away from this is remember, if they give you morality of a of a compound or solution, it just means that number in terms of moles divided by 1 kg of our solvent. This will be important to remember if we're trying to convert morality to, let's say, mole, Arat e or Mole Fraction or even mass percent. Now that we've learned the basics of morality, click on to the next video and take a look at odds morality.

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concept

## Osmolality

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Alright, So Ionic Morality or Oz morality represents the morality of dissolved ions within a solution. Now, for example, if we're given 0.30 Molo off sodium chloride, sodium chloride associates into sodium ion and chloride ion, that's a total of two ions involved. If we want the oz morality that's equal to the number of ions, which we said was too times the morality of the compound, which is 0.30 Molo, this will give us an Ionic morality or Oz morality of 60. So remember, when we're talking about Oz morality or ionic morality, we have to take into account the number of ions to disassociate within our given solution. Now that you've seen this, move onto the example left on the bottom of the page and see if your answer matches mine.

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example

## Calculate Molality Example

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Ah, solution is prepared by dissolving 43 g of potassium chlorate and enough water to make 100 ml of solution. If the density of the solution is 1.760 g per mil. Leader, what is the morality of potassium chlorate in the solution? We're told the molecular weight of potassium chlorate is 1 22 55 g per mole. All right, so we need to determine the morality. That means we need moles of potassium chlorate divided by kilograms off our solvent. From the given information, we have 43 g of potassium chlorate. We have 100 mls of solution. We have the density of our solution, and we have the molecular weight of potassium chlorate. Now, the molecular way to potassium chlorate can be used to change our grams of potassium chlorate given into moles. So we're gonna stay here for every one mole of potassium chlorate. We're told the masses 1 22 55 Gramps. So grams, cancel out. And now we're gonna have moles. So when we do that, we get 0.350877 moles. So that equals 0.350877 moles here and now we have to determine what our kilograms of solvent are. The only other piece of information that we have left to use is the fact that we have the volume of our solution and the density of our solution. Realize now that we can use these to weaken, multiply them together, to have the volumes cancel out. So when we do that, that gives me 176 g of solution. Now, remember what is in a solution? A solution is made up of Salyut plus solvent together. We already know how many grams of saw you we have so we can subtract it out and realize that when you subtract out the grams of solute, we'll have left our grams of solvent. Here really isn't important what the identity of the solvent is. But it is water because we said that this was in in water. All right, so now we just have to change those 133 g into kilograms. So for every 1 kg, it's 1000 g. So that's 0.133 kg. Take that and plug it here below the moles and we'll have our morality as being two point mol for potassium chlorate solution. So, like we always do, we approach this like a dimensional analysis question. We write down with the information they're asking us to find first. Then we write all the given information from there. We just need to manipulate it and arrange it in a way toe. Isolate the variables that we need, which in this case, was to help us to find morality. So keep this in mind whenever facing any questions, dealing with determining the morality of a solution or an ion.

Molality Calculations

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example

## Molality Calculations Example 1

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if the morality of glucose in an aqueous solution is 2.56 What is the mole? Arat e Here they tell us the density of the solution is 1.530 g per mil leader. All right, so we're looking for the polarity. Similarity here would equal moles off our glucose, divided by leaders of solution. The given information is we have 256 Molo of glucose, and we're also given the density of the solution is grams of solution per 1 mL of solution. All right, so we have to expand out on morality. 2.56 Mohler, What does that really mean? Well, that's equal to 2.56 moles of glucose per 1 kg of solvent, which in this case would have to be water because it's an aqueous solution. Acquis means water. Right now we can see that we have already the moles of glucose, which we can plug in here. So we have 256 moles of glucose which you can plug up top Now we need to figure out what our leaders of solution are leaders of solution. We don't have it directly. We have milliliters off solution instead. All right, To get milliliters of solution isolated, we need to cancel out grams of solution up here. To do that, we're gonna have to take the moles of glucose and change them into grams. So we're gonna say here that 256 moles of glucose when you add up all the carbons, hydrogen and oxygen, the combined weight is 181 56 g as the mass of glucose. And that's for every one mole of glucose moles cancel out will have grams, which comes out to 4 61. g. And also we have 1 kg of water which is equal to 1000 g of water. So here we have our solvent. Here we have our Salyut. When we add them together, that will give us our grams of solution. Because remember, a solution is saw you plus solvent together. Now that we have grams of solution, we can multiply it with the density. So we're gonna say 1461.199 g of solution times the density put the grams here on the bottom so we can cancel out and then one millimeter on top. Then remember, For every 1000 mL, we have one leader. So grams of solution cancel out milliliters. Cancel out. We'll have leaders left at the end, which comes out 2.955 leaders. So take those and plug him here on the bottom. So that gives us a final polarity of two point 68 Mueller. So with this example, we're going from polarity. What we're going from morality tomb Olara tea, with the use of the density of our solution again, is always write down the information they asked us to find first, then manipulate to given information to get your desired answer at the end. Now that we've done this example, move onto the second one and see if you can get the correct answer. Come back is always and see if your answer matches up with mine.

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example

## Molality Calculations Example 2

1m

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So here it says, What is the ionic morality off nitrate ions in point three or five mobile off lead for nitrate. Alright, so here we're just looking for the mortality of our nitrate ions. Here lead for nitrate breaks up into one lead for ion plus four nitrate ions. So the Ionic morality or Oz morality would equal the four nitrate ions times the morality of the entire compound. So that comes out to being one point two two Molo. So just remember, with ionic morality, they can ask you for the total ionic morality which takes into account all the ions, so that would be five ions. Total time is the morality of the compound. Or they can ask for the ionic morality of individual ions. In this case, there are four nitrate ions. So we multiply the total mortality by four to get our final answer. Now that we've seen examples one and to tackle the practice question left on the bottom of the page,

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Problem

What is the mass percent of NH_{3} of a 1.25 m aqueous solution of NH_{3}?

A

1.11%

B

3.53%

C

2.08%

D

87.4%

Additional resources for Molality

PRACTICE PROBLEMS AND ACTIVITIES (1)