2. Tools of the Trade
Buoyancy
Struggling with Analytical Chemistry?
Join thousands of students who trust us to help them ace their exams!Watch the first video
Video duration:
3mRelated Videos
Related Practice
Skip to main content
pipettes that you'll be using in your analytical labs come from the manufacturer as usually Class A pipettes. But once you've obtained them, there is some level of uncertainty associated with them. You have to first calibrate them before we can use them in experiments. Here, we're going to say convenient method in calibrating pipettes is to weigh the water delivered from them. By using the density of water at a given temperature, we can determine the volume of density with greater accuracy. So here it says assume a 50 milliliter pipette is in need of calibration. An empty flask weighs 49.563 grams. When water delivered from the pipette is added to the empty flask, the new mass is recorded as 69.618 grams. We're asked, What is the mass of water delivered? Now, here they're saying the density of the standard weights is determined to be 8.40 grams per milliliter. Remember, in the equation above, we said that the standard weight is 8.0 grams per milliliter. In this question, we must be using a different type of metal or alloy for those calibration weights which is why it's a new number. Again, always be careful if they're giving you a new density for the calibration weights if they do use that one. We're going to say here m equals m prime times 1 minus d a over d w divided by 1 minuteus dA over d. We need to first find the apparent mass. Well, when the flask is filled with water, the weight of the water and the flask together is 69.618 grams. When the flask is empty, it weighs that much. When we subtract them, that'll give me the mass of just the water. So that comes out to 20.055 grams. So that's our apparent mass. So times 1 over, So DA is the density of air, which is 0.0012 grams per milliliter. The density of the calibration weight, we're told, is 8.4 0 grams per milliliter divided by 1 minus density of air again divided by the density of water. Here, they don't tell us the exact temperature. The density of water can change with changing temperatures, but most of them are around 1 gram per milliliter. We're going to use 1 gram per milliliter for the density of water. We just have to solve for our true mass now. So that's 20.055 grams times when I work all this out in here, it gives me 0.99857. Divide by the bottom when I subtract this value here from 1. What does it give me? It gives me 0.998. Then what do we get at the end? So at the end, we're going to get 20.079 grams. So notice our apparent mass when we read it from the analytical balance was that number, but the true mass when we take into account airflow gives us this new mass for our water. Again, this is a great way in order to find the true mass of an object and also a good way to calibrate the pipettes that you'll be using extensively within your analytical labs. So now that we've seen this, we'll take a look at buoyancy when it's found within fluids.
03:02
