In chemistry, a liquid solution is a number of solids dissolved in a liquid. For example, tea is a solution consisting of dissolved tea molecules and water.
A homogeneous solution is one where the dissolved molecules are evenly distributed throughout the solution i.e tea. A heterogeneous solution is one where the dissolved molecules are not evenly distributed. Think about soda vs. vegetable stew. Every sip of soda you drink is more or less the same in texture and taste, whereas every spoonful of vegetable stew may have a different ratio of vegetables. We can say that the soda is homogeneous whereas the stew is heterogenous.
The solute is whatever's being dissolved, and the solvent is whatever the solute is being dissolved in. Tea is the solute with water as the solvent. Most of the time in chemistry, water will be the solvent. Water is commonly referred to as the universal solvent because of its ability to dissolve more substances than any other known liquid.
Water fortunately doesn't dissolve everything. If it did, the entire world would be one large solution (think about it). In fact, you can see the limitations of water's capabilities as a solvent in your own kitchen! If you were to mix oil and water, the two would separate into two distinct layers as the water is unable to dissolve the oil. This is because of the polarity of the two is different. In general, polar substances will dissolve polar substances and non-polar will dissolve non-polar. This principle is often abbreviated to like dissolves like. Water is unable to dissolve oil because water is a polar molecule whereas oil is a non-polar molecule. Similarly, non-polar molecules like hexane are insoluble in water but soluble in other non-polar liquids, such as methanol or ethanol.
Molarity
Molarity(M) is defined as `"Moles of Solute"/"Liter solvent"`. This is just a fancy way of saying "the amount of something in 1 liter of water (assuming water as the solvent)." For example, if I had 5 moles of table salt (NaCl) molecules dissolved in 1L of water, I would have a solution that is `(5 "moles NaCl")/"Liter"`, or 5 molar NaCl. Molarity designates the concentration of a solution by indicating how much of the solute is dissolved in the solution.
#1. Calculate the molarity of a solution consisting of 3 moles of glucose in 5L of water.
This is a pretty straightforward problem. The molarity is defined as `"moles solute"/"Liter solvent"`. In this case, glucose is the solute and water is the solvent. The calculation is therefore:
`"Molarity of Solution"="moles glucose"/"L water"="3 moles"/"5L water"=0.6M`
We say this solution is 0.6 molar, written as 0.6M.
Answer: `0.6M`
#2. How many grams of Hydrochloric acid (HCl) is in 400mL of a 2M solution?
This problem requires us to use molarity as a conversion factor. The first step is to find out how many moles of HCl are in the 2M solution.
`(400 "mL")((1 "L")/(1000 "mL"))((2 "moles HCl")/(1 "L"))= 0.8 "moles HCl"`
Now that we have moles, we just have to convert to grams. The molecular weight of HCl is `(36.5 "g")/"mol"`.
`(0.8 "mol HCl")((36.5 "g")/"mol")="29.2 g HCl"`
Answer: `29.2 "g HCl"`
#3. How many liters of water are necessary to dissolve `30 "g"` of `CaCl_2` to create a 0.4M solution?
This question asks essentially the same thing as the previous question. The only difference is that we're now solving for `L`.
`("30 g" CaCl_2)("1 mole"/("111 g" CaCl_2))((1 L)/(0.4 "mol"))= 0.68 L`
Answer: `0.68 L`
The significance of molarity is that it allows us to consistently define the concentration of a solution. A 3M solution of `MgCl_2` will be the same no matter where you are in the world or how many liters of the solution you have. Sometimes you may only need 20mL of a solution whereas other times you may want as much as 50L. Molarity provides a simple way to calculate the relative amounts of solute and solvent necessary to make create the exact concentration of solution necessary. Additionally, molarity allows us to see how potent a solution is. For example, a 10M solution of HCl is significantly more dangerous to handle than a 0.1M HCl solution is; the molar amount is 100 times as much! Molarity is one of the most fundamental concepts of chemistry and we'll be using it consistently throughout future posts, so make sure you understand it!
Dilution
Sometimes you're going to have a solution that has a higher molar concentration than you'd like. What do you do in that case?
The simplest solution (pun unintended) is to dilute the initial solution. If you've ever mixed cold water into water that was too hot in order to bring down the temperature, you've performed a dilution. In terms of molarity, diluting a solution means to bring down the concentration. This is accomplished by increasing the amount of solvent.
In order to find the concentration of the new solution, we use this equation:
`M_1V_1=M_2V_2`
`M_1`= Molarity of solution 1
`V_1`= Volume of solution 1
`M_2`= Molarity of solution 2
`V_2`= Volume of solution 2
Note that the 1 and 2 don't actually mean anything. You can choose either solution to be 1 or 2. The important thing is to stay consistent: all subscripts 1 refer to the same solution, be it for molarity or volume.
#4. Calculate the molarity for 1L of a 5M solution after 500mL of water has been added to it.
Set `M_1=5M`, `V_1=1 L`, and `V_2=(1 L+0.5 L)`. `M_2` is what we're trying to solve for.
`M_1V_1=M_2V_2`
Divide both sides of the equation by `V_2` in order to isolate `M_2`
`M_2=((M_1V_1)/V_2)`
Now you can solve for the molarity of the diluted solution by plugging in the values of `M_1, V_1`, and `V_2`
`M_2=((5M)(1L))/(1.5L)=3.33 M`
Answer: `3.33 M`
This is what we expected. By increasing the volume, the molarity decreases. Effectively what we are doing is increasing the denominator in the `"Moles"/L` equation.
Notice that the number of moles remains the same before and after dilution. The number of moles doesn't change unless we add more of the solute into the solution.
#5. You have 50mL of an 8M solution and want to dilute it so that you have a 0.75M solution. How much of the 0.75M solution will you have?
The process is nearly identical to the previous problem. The only difference is that this time we're solving for volume.
`M_1V_1=M_2V_2`
Divide both sides by `M_2` to isolate `V_2` on one side.
`V_2=((M_1V_1)/M_2)`
Now we can just plug in values for `M_1`, `V_1`, and `M_2`!
`V_2=((8M)(0.05L))/(0.75M)= 0.53L`
Answer: `0.53 L`
Solubility
The last property we're going to cover in this post is actually a property of the solute. The solubility of a substance is defined as the amount of the substance that will dissolve in the solute. For example, we can say that table salt has a solubility of `(35 "g")/(100 "g" "water")`. This means that the maximum amount of salt that will dissolve in 100g of water will be 35g.
This solubility is sometimes also called the solubility limit. Once a solution has too much of the solute dissolved in it, the solute will simply fail to dissolve and the solution becomes saturated. There will also be some substances that will simply fail to dissolve, no matter how little of it is used. These substances are deemed insoluble in the solution. An example is AgBr, silver bromide, which is insoluble in water. If you've ever made Kool-Aid using too little water, you'll notice tiny amounts of the Kool-Aid powder present at the bottom of the pitcher. This is because the Kool-Aid has reached its solubility limit and cannot dissolve any further.
Summary
1. A solution consists of a solute and a solvent. The solute is what's being dissolved whereas the solvent is what the solute is being dissolved in.
2. A homogeneous solution is one in which the solute is evenly distributed throughout the solution. A heterogenous is one in which the solute is unevenly distributed.
3. In general, polar solvents will dissolve polar molecules and unpolar solvents will dissolve unpolar molecules. This is colloquially summarized as "like dissolves like."
4. The molarity of a solution is the concentration of the solution and is defined as `"moles solute"/"L solvent".`
5. In order to dilute a solution, you need to increase the volume.
6. Solubility describes how much of a substance will dissolve in a solution.
Fun Facts
1. Molarity of Water
Usually when we talk about molarity, we talk about the molarity of some solute dissolved in water e.g `0.5M "HCl"`. It turns out water has its own molarity! It's a weird idea to think about, but molarity is just defined as the number of moles per liter. In `1 "liter"` of water, there is approximately `55-56 "moles"` of water! Therefore the concentration of water is `55M`.
2. What kind of bear is soluble in water?
A polar bear!