Water is oh so special, being the star of this episode! Episode 19 starts with a recap of Episode 8 definition of solution components, electrolytes and concentration.
Water is oh so special, being the star of this episode! Episode 19 starts with a recap of Episode 8 definition of solution components, electrolytes and concentration (1:27). Concentration can also be determined relatively by using particle diagrams (2:35). Another application of particle diagrams is to visualize the interaction between solvent particles and solute particles, for example between water and NaCl (4:27). But what about water and oil? (5:21). The difference in boiling point between components of aqueous solutions can be used to separate them during distillation (6:17).
Question: (7:55) If you combine 2 liter of 1M NaCl and 2 liter of 1M KF solutions in a 4 liter container, what would be the new concentration of each ion?
A.Helium B.Fluorine C.Oxygen D.Xenon
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Hi and welcome to the APsolute Recap: Chemistry Edition. Today’s episode will recap Aqueous Solutions.
Lets Zoom Out:
Unit 3 - Intermolecular Forces and Properties
Topics 3.7 - 3.10
Big idea - Scale, Proportion and Quantity
Our hype about water continues: an episode all about aqueous solutions! When we hear the term solutions, we often automatically think about aqueous solutions: saltwater, diluted bleach, soda, rain, coffee and so many more! But, when I say we are constantly surrounded by solutions, then I do not only mean the examples I’ve just mentioned. Air, for example, is also a solution. Surprised? In chemistry, solutions are defined as homogenous mixtures and can be solids, as inthe form of alloys, liquid, like aqueous solutions or gaseous. However, in chemistry, we do often have aqueous solutions. Therefore, today’s episode recaps aqueous solutions.
Let’s zoom in:
We’ve already talked about solutions in episode 8 - so let’s recap the recap in light of today’s episode: in aqueous solutions the component that is most abundant is water - also known as the solvent. The component that is less abundant and is dissolved is the solute. The solute can be a solid, liquid or gas. Our tip to remember this is: “water is the uniVersal solVent”, both with a V. Solutions that conduct electrical current are called electrolytes. The opposite would be nonelectrolytes. The concentration of an aqueous solution is measured in Molarity. The unit has a capital M and means moles per liter of solution. So if you want a 1 molar solution of NaCl, you have to dissolve 58.44 g of NaCl in approximately 1 liter. Why approximately? Because the unit is per liters of solutions. You have to take into account that the NaCl has a, admittedly rather small, volume itself. Therefore, we use volumetric flasks to mix solutions.
Using the unit moles/liter, we can shift to the particulate view of solutions, which we use to describe the interaction between the water molecules and the solute molecules as well as to describe concentration. Let’s start with the concentration: as we know, one mole contains 6.022 x 1023 elementary units - in our case - units of the solute. You will not be asked to draw 6.022 x 1023 particles (phew!), BUT: you should be able to relate the number of solute particles in two different containers with the same volume to concentration. If container number 2 has twice as many particles, then the concentration is twice as high! Actually counting the particles in those particle diagrams is really important! Never forget: Matter can neither be created nor destroyed!
The second application of particle diagrams often relates to the interaction between water molecules and solute. Don’t get salty with me, but let’s stick with NaCl and recap: water is a polar molecule with a partially positive charge on hydrogen and a partially negative charge on oxygen. This polarity allows water to dissolve or dissociate many substances: its the universal solvent. When NaCl dissolves in water, the partially positive hydrogens in the water molecules are attracted to the chloride anions. At the same time, the partially negative oxygen within the water molecule is attracted to the sodium cations. The water molecules surround the crystal lattice respectively. The attractive forces between the ion and the dipole are strong enough to overcome the hydrogen bonding between the water molecules.
Now, this wouldn’t be chemistry if there wasn’t a caveat to it. And there is - as in with the form of oil, for example. We know that oil and water are like uhm… oil and water or like iron man and thanos or orange juice and toothpaste! Water and oil are immiscible because they lack intermolecular interaction. We use the term miscibility when talking about two liquids. Water is a polar molecule, oil is nonpolar. Oil and water do not have interactions that are strong enough to overcome the hydrogen bonds between water molecules. And so the water molecules would rather stay amongst themselves! You often hear the phrase “like dissolves like” - it is a great guideline BUT not AP level. In the exam you need to be able to EXPLAIN why a substance does or does not dissolve or dissociate using the polarity of molecules and intermolecular interactions.
The polarity of water can also be used to separate aqueous solutions, as with distillation. During distillation you are taking advantage of the different boiling points of the components, which are determined by the intermolecular forces! So let’s boil an aqueous solution of ethanol: We have hydrogen bonding between the water molecules, and because of the OH group, also between the ethanol. However, there are less hydrogen bonds in ethanol due to the carbon chain. We also have hydrogen bonds between the water and the ethanol. These are the forces we have to overcome when transitioning from liquid to gas. As we know, water has a boiling point of 100 degrees Celsius, which is unusually high due to the hydrogen bonding. Ethanol has a boiling point of 78 degrees Celsius. If I heat up the mixture to around 78 degrees Celsius, the majority of molecules entering the gas phase will be ethanol. In our distillation apparatus, the gaseous ethanol will then be cooled and collected as liquid. The remaining liquid is then water. Tada!!! Separated!
To recap:
In aqueous solutions, water is the solvent. Concentration of aqueous solutions if often expressed as molarity or moles per liter of solution. Particle diagrams can be used to determine relative concentrations as well as show interactions between solvent and solute. Solute-solvent interactions determine if a solute is soluble or miscible. Distillation can be used to separate aqueous solutions by utilizing the different boiling points.
Coming up next on the APsolute RecAP Chemistry Edition: Types of Chemical Equations
Today’s Question of the day is about concentrations of solutions.
Question: If you combine 2 liters of 1M NaCl and 2 liters of 1M KF solutions in a 4 liter container, what would be the new concentration of each ion?