Hi there, everybody! My name is Jeremy Krug. Welcome to my ten minute review of AP Chemistry Laboratory Skills. Although this video can’t replace a high-quality lab program, we’ll review and hit the main points you need to know for the AP exam in this video. If you learn something from this video, smash that LIKE button, and share this video with the other members of your AP Chem community! And don’t forget that if you want the FULL AP Chemistry course review, along with hundreds of practice questions, full unit summary videos, and guided notes to my videos, head over to UltimateReviewPacket.com and check out my FULL AP Chemistry review resources, everything you need to succeed on the big exam in May! Let’s get started…. In the lab, you have to know how to work safely, wear your safety goggles, never touch chemicals with your fingers, only add concentrated acid to water – never the other way around. If you have an acid spill, you neutralize it with weak base, like baking soda. Likewise, if you have a base spill, neutralize it with a weak acid, such as vinegar. You never neutralize a solution with a strong acid or base, because if you add too much, then you have something that’s still strongly acidic or basic. Know the colors of some ionic solutions. Copper ion solutions are blue, iron ion solutions are yellow or orange, nickel ion solutions are green, cobalt ion solutions are pink, and chromium ion solutions are yellowish or orange in color. Each piece of lab equipment has its own specific purpose. If you need to dispense a very specific volume of liquid or solution, you will use a volumetric pipette, usually just called a pipette. Pipettes are available in 1 and 5 milliliter varieties, but the most common ones used in AP Chemistry are 10 milliliters and 25 milliliters. If you’re dispensing a solution and then need to measure precisely how much volume has been dispensed after the fact, you use a buret. Most of the time, we use a buret for measuring how much solution has been dispensed in a titration. Notice that zero on a buret is at the top, so if you’re new at using one, it almost feels like you’re reading it backwards. When you read a buret, or any volumetric glassware, always estimate exactly one decimal place between the lines. So this buret would be read as 15.45 milliliters. Notice that there’s a curvature here, called a meniscus. Always read the meniscus at the vertex of that curvature. A graduated cylinder can be used for dispensing a somewhat precise volume, accurate to within possibly a fraction of a milliliter. Graduated cylinders are not precision instruments though! Now, the least precise measuring instrument for liquids is the beaker. Beakers are just glorified cups. We use beakers just to contain a liquid or a solution, never to measure it. The volume marks on a beaker are very approximate and shouldn’t be used as a precise measurement for anything! You need to know the different types of flasks in the laboratory. A Florence flask is basically just a storage container for a solution, or commonly for distilled water. It’s usually round and normally doesn’t have any measuring marks on it, so it’s strictly for containing a liquid. An Erlenmeyer flask, on the other hand, has this narrow neck on it. We use Erlenmeyer flasks so we can swirl the solution around inside and shake it up a little, without the contents splashing out. Erlenmeyer flasks are placed underneath a buret when we perform a titration. Volumetric flasks are used to produce solutions with a very precise concentration. To make a solution using one of these, very carefully transfer the desired amount of solid into the flask, then add some water to get the solid to dissolve. Once all the solid is dissolved, you add more water until the bottom of the meniscus is flush with the line on the volumetric flask. That line represents the flask’s capacity – it might be 100 milliliters, maybe 500, that’s the volume we’re working with. Then, to distribute the solution evenly, we put a cap on the flask, then invert the flask and shake it ten times. The AP Chemistry exam writers assume you’ve done several chemistry experiments in the lab. Every lab experience is a little different, so the experience at one school is a little different than at another school, and that’s okay! However, there are some classic experiments that you need to understand how to do. Acid-base titrations are absolutely required. If you’ve never done an acid-base titration from start to finish, watch my video on that. Here are a few key points. Use this equation – what I call the titration equation – to solve for the missing molarity or volume in a titration. At the equivalence point, the molarity of the acid times the volume of the acid equals the molarity of the base times the volume of the base. If you know any three of those variables, you can solve for the other one. The most common titration in AP Chemistry involves using a volumetric pipette to dispense a certain volume of an acid into an Erlenmeyer flask. You add a few drops of an acid-base indicator that will change color when you get to the endpoint of the titration. Then you use a buret to dispense a base – usually a strong base like sodium hydroxide – into the acid solution. When the indicator changes color, you know you’re at the endpoint, and you can use the volume of base added from the buret in your calculations. I have a much more detailed discussion of titration curves in my full course video, but you need to know that the pKa – or negative log of the Ka of the weak acid – is equal to the pH on your titration curve halfway to the equivalence point. And this inflection point represents the equivalence point. You should know how to separate mixtures. Know the difference between filtration, distillation, and chromatography, and when you’d use each one. Filtration is used to separate a solid from a liquid. So if you have a solid, such as a precipitate, mixed with some other aqueous solution, you could use filtration to isolate the precipitate. Distillation is used to separate liquids that have distinctly different boiling points. So if you have a mixture of alcohol and water, you could separate those using distillation, since alcohol boils around 78 degrees Celsius, and water boils around 100 degrees. Chromatography is usually used to separate relatively small amounts of different components in a solution. Column chromatography involves pushing a solution through a fairly long column, knowing that the different components of the solution will fall through the column at different rates. In paper chromatography, we take a dot of a solution, possibly ink or something similar, and we place it close to the bottom of a piece of special chromatography paper. Then we dangle the bottom tip of the paper into a separating liquid. As the liquid rises up the paper, the different components of the mixture rise up the paper at different rates. Any component of the mixture that has very similar intermolecular forces to the separating liquid will be very heavily attracted to it, and that component will rise up the paper the fastest. On the other hand, any component that has intermolecular forces that are very different from the separating liquid won’t be attracted to it very much at all, and that component will rise up the paper the slowest. Eventually, the different components will be separated quite a bit from each other, and the longer you wait, the farther apart the components will be. Spectrophotometry is a lab technique that pops up on every AP Chemistry exam. I have two videos in my full course playlist dedicated to this, but the main idea behind spectrophotometry is that first, we select a wavelength that has the highest absorbance for that particular ion, while minimizing interference from other substances that might be present. We take several samples of known concentration and determine their absorbance, and use that data to create a calibration curve – a line that plots concentration on the x axis and absorbance on the y axis. Then, we take an unknown solution, and once we get its absorbance, we line it up to the calibration curve and use that to estimate the concentration of the unknown. You’re expected to know the effect of errors in an experiment. Most of the time, simple algebra will be the key to figuring these out. For example, in spectrophotometry, we use the Beer-Lambert Law, which is written A equals epsilon b c. If we contaminate the cuvette with water, just as an example, that means the actual concentration is lower than it should be. Since c, concentration, is too low, that means that A, absorbance, is also going to be too low, and we see that just by looking at the equation. If you’re doing an experiment with gases, and you’re using the ideal gas law, PV=nRT, if you have a faulty thermometer that reads the temperature too high, and you’re trying to solve for number of moles, n, that means your calculated value for n will be too low. Use the algebra in the equation to figure out the effect of an error. You’re sometimes asked to calculate percent error. The percent error of a result is equal to the absolute value of the difference between the calculated answer and the correct answer, all divided by the correct answer, multiplied by 100. That’s my ten-minute laboratory review! Thanks for joining me, and don’t forget to watch my other review videos! And don’t forget to get ALL of my review resources over at Ultimate Review Packet dot com! If this video’s been helpful to you, please smash that like button and leave a comment! I wish you all the best as you review for the big AP Exam, thanks for watching, and I’ll see you next time!