Outline the steps required for conversion between molarity, molality, and percent by mass.

Name: Problem 14, Chapter 12: Chemistry
Uploaded: 2021-03-04T18:06:20-08:00
Duration: 10 min 37 s
Channel: Noah Boudrie
Description: Outline the steps required for conversion between molarity, molality, and percent by mass.

Outline the steps required for conversion between molarity,...

Key Concepts

Molarity

Molarity is a concentration unit defined as the number of moles of solute per liter of solution. It provides a volume-based measure of concentration and is often used in reactions and processes where solution volume is a critical parameter. Conversions involving molarity require careful consideration of the solution’s overall volume and any changes due to temperature or mixing with other substances.

Molality

Molality is defined as the number of moles of solute per kilogram of solvent. Unlike molarity, it is independent of temperature since it is based on mass rather than volume. Molality is particularly useful in situations where temperature fluctuations might affect volume measurements, such as in colligative property calculations.

Percent by Mass

Percent by mass (also known as mass percent) is a concentration measure that expresses the mass of the solute divided by the total mass of the solution, multiplied by 100. It provides a direct way to express the composition of a mixture in terms of weight and is commonly used when the masses of solute and solution are more directly measurable than their volumes.

Density

Density, defined as mass per unit volume of a substance, is crucial when converting between volume-based and mass-based concentration units. It allows for the interconversion between molarity (which uses volume) and molality or percent by mass (which use mass), bridging the gap between the different concentration measures in a solution.

Molecular Weight

Molecular weight is the mass of a given substance (molecule) and is used to convert between moles and grams. It plays an essential role in converting concentration units since it enables the calculation of the mass of the solute from its amount in moles, facilitating transformations between molarity, molality, and percent by mass.

Transcript

00:01 Particular problem, we are asked to outline the steps required for conversion between different ways of measuring, different ways of measuring concentration.

00:09 And the three different ways we're asked to outline the steps for converting between.

00:14 It's going to be molarity, which i have here in red.

00:17 This is going to be the moles of solute divided by the liters of solution.

00:22 What this is conceptually is, if we're thinking about an example where we're sweetening some tea, we're putting some sugar in some tea.

00:28 The sugar put in the tea is going to be the solute.

00:31 So we need to know how much of this sugar exists in the entire solution.

00:37 That's going to be the liters of solution.

00:39 The solution here would be the sweetened tea.

00:41 So it would be including everything in this cup.

00:43 This would be the solution.

00:44 So you need to know the volume of the solution and the amount of sugar that is present.

00:49 Now molality, this is in blue.

00:51 And it's a very similar concept.

00:53 But the difference is if we have the same example, we're looking at sweet tea.

00:58 The moles of solute is again going to do the sugar that is put in the tea to make it sweet.

01:03 But the mass of the solvent, the solvent is just going to be the unsweetened tea.

01:09 So it's not going to be everything included.

01:12 It's not going to be the sugar and the tea.

01:13 It's just going to be the tea.

01:15 And we need to know the weight of the tea in kilograms.

01:19 So that's going to be molality.

01:21 And percent by mass is just showing if you were to know the entire weight of a solution.

01:27 So the mass of the solute and the mass of the solvent because the entire solution is made up of these two components.

01:33 We're looking at how much of the weight the solute is responsible for.

01:38 So if we were to weigh the sweetened tea, how much of this weight is the sugar versus the actual tea in its entirety.

01:48 So now let's see how we would convert between these two units, or these three units actually.

01:53 So let's first focus on molarity to molality.

01:58 And let's just outline the steps for this.

02:00 So first of all, we need to have the malaria.

02:05 This is very important.

02:06 So we need to have molarity, just kind of a no -brainer.

02:13 But again, if we have the things we need to know to solve the malarity, then we can do that.

02:19 But if we don't have it, this would be a great place to start.

02:22 So the next step is to convert liters to kilograms using density.

02:43 Because density, the units are going to be, you know, let's make this a little cleaner, grams per liter.

02:58 It's the amount of stuff that exists in a given space, right? so it's mass per volume.

03:08 So it's another way of thinking about concentration.

03:11 The density is great for that.

03:12 So we need to convert the liters of solution to total kilograms because when we're looking at molality, we need to know things in terms of kilograms, right? we see kilograms here and then liters here.

03:32 So we need to convert between volume to actual mass.

03:37 So that's the next step.

03:40 The third is converts the solute from moles to grams, well technically kilograms, moles to doesn't really matter there's just there's a certain amount of preference that goes into this particular problem just clean this up a little bit convert solute from moles to kilograms and then we need to have something to bridge the gap for step two it was density for step three it's going to be amu and you can if it's just an element that the solute so if the solute is just an element you can use the periodic table which i guess you'll have to use if it's a compound or a molecule, you'll have to combine a few different amus of whatever elements are actually in the solute.

04:47 So step four is you'll need to subtract the mass of solute, subtract mass of solute from mass of solution.

05:16 Because when we convert, in step two, when we convert liters to kilograms, this is of the entire solution.

05:24 And the solution is including the solutes and the solvent.

05:27 When we're looking at molality, we need to find the mass of the solvent.

05:30 So we need to, again, break things into the components.

05:33 So first we have to convert units to get to kilograms, find out how much of this, how much of actual solute is existing by converting from moles to kilograms, and then we can subtract this from the actual mass of the solution.

05:53 And then now we will have the mass of solvent...

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