Chemical Composition

Percent Composition

We can take molar mass and break compounds down into their individual elements to determine its percent composition, or how much of each element it's made of. To do this, we take the mass of each piece of the compound and divide it by the molar mass, then turn it into a percent.

Solutions

If 2 substances can mix and make a homogeneous mixture, they are known as soluble with one another. Otherwise, they are insoluble. A more specific form of this is miscibility, which only applies to liquids when they can completely mix with one another. If something doesn't dissolve into a liquid it can still float around within it, forming a suspension (think sand or flour floating in water).

When dissolving one material into another a solution is formed. The material that is being dissolved is known as the solute while the material doing the dissolving is known as the solvent. Solvents have a certain amount of particles that can dissolve into them - when they have that exact amount of particles the solution is known as saturated. If it has less than that amount, it's known an unsaturated. If it has more, it's known as supersaturated (and would have solid floating around in it). The amount of solute that can dissolve into a solvent and the speed at which it does so generally increases with increasing temperature due to how much the particles within the solute can move.

Some liquids are measured in percent by volume or percent by mass instead of Molarity. This amount is simply a ratio of how the amount of solute compares to the total amount of solution, either by mass or by volume:

% by Volume = Volume of Solute / Volume of Solution x 100

% by Mass = Mass of Solute / Mass of Solution x 100

Trying to go from percent compositions to a formula takes a bit more effort but it can be useful to figure out what molecule you are specifically working with. There a few steps to follow for this process:

Turn the % symbols into grams
Divide each amount of grams you have by the molar mass of that particular element
Divide each of these numbers by the smallest number you got from this process. This gives you to subscripts for each element:

Molarity

Concentration of solutions is measured in Molarity (M) by taking the moles of the solute and dividing by the liters of solution:

The larger the Molarity, the more concentrated the solution.

Concentration can be modified in a few different ways:

Adding Solute: Adding more solute increases the molarity of the solution.

Molarity = (original moles + moles added) / Liters

Adding Solvent: Adding more solvent is also called dilution. This process lowers the concentration of the solution.

Initial Molarity x Initial Volume = Final Molarity x Final Volume

Adding another Solution: When two solutions have different concentration and are mixed, the overall concentration ends up somewhere in between the molarities of each of them.

Total Molarity x Total Volume = Molairty of First Solution x Volume of First Solution + Molairty of Second Solution x Volume of Second Solution

Separation: Separation can be used to split the solute from the solvent, even though it's mixed. There are many ways to separate solutions. Some examples include:

Distillation: Separation by phase changes. This is usually done through boiling and condensing.
Centrifugation: Separation by density through spinning. Once spun the heavier substance can be skimmed off of the lighter substance.
Chromatography: Separation by applying a different medium to a mixture. There are many different mediums that can be used, such as gases, liquids, and even paper.
Filtration: Separation by particle size. Usually this is done through filter paper, grates, or something of the like.

Concentration and pH

pH is based on the concentration (denoted by an ion found within brackets) of hydronium (H3O+) ions in a solution, which are produced when an acid (of any kind) is dissolved in water. To find it from concentration we use the following formula:

pH = -log[H3O+]

Doing the reverse and trying to find concentration from pH uses the following:

10-pH=[H3O+]

The reverse of this scale is known as pOH and is based on the concentration of hydroxide (OH-) ions. While not often used, pOH is useful for calculating the pH of bases since pH and pOH always add up to 14.

pOH = -log[OH-]

Since it adds up to 14 this also means the concentrations should always multiply out to 10-14 M.

IVs and Dissolved Medications (PSA)

When making an IV there's a lot of conversions based on the medication being used. Generally we start with a saline (salt) solution of 0.9% (0.15 M) sodium chloride, then add medicine into the bag if necessary. This concentration is used because it's similar to the concentration of salt found within many fluids in the body.

When adding medication to an IV drip the amount of medication, size of the IV tubing, and how long to administer the medication for is all needed. Once these are known an IV's drops per minute can be calculated using dimensional analysis:

? Liters / ? Hours * (1000 mL/L) * (? drops/mL) * (1 hr/60 min) = ? drops / min

To find the concentration of a medication it works just like the other mole conversions. Convert the grams (or milligrams) into moles based on the medication's molar mass, then divide by the liters of solution.

Additional Resources

Interesting Links

Page updated

Report abuse