Weigh agarose using tare and a weigh boat
Zero the balance with the boat already on it, land inside the tolerance, then rinse the boat so the powder you weighed is the powder that arrives.
- A container has a mass of its own: Anything you put on a balance is weighed, including the thing holding the sample. Students who have only weighed objects directly do not expect the container to be part of the number.
- A tolerance is a band, not a target: Plus or minus a stated amount defines a range of acceptable values. Reading it as a single number makes students chase a digit that does not matter.
Prerequisites are inferred: pending teacher review.
Re-learn the skill with worked practice and clear examples.
Weighing it right is only half the job. Now all of it has to reach the flask.
Why does the second measure of buffer go into the weigh boat before it goes into the flask, rather than straight into the flask?
Reviewed- A.It lets the buffer pick up the tare setting
- B.It checks that the boat is clean for the next group
- C.It cools the buffer before it meets the powder
- D.It rinses the clinging powder in with it
Show the worked solution ▾
Answer: D. It rinses the clinging powder in with it
- Step 1: Ask what is left in the boat: Powder that clung to the sides when the boat was tipped out.
- Step 2: Ask where that powder is supposed to be: In the flask, since it was counted in the mass on the balance.
- Step 3: Ask what a liquid does to clinging powder: It carries it along, and this liquid is going into the flask anyway.
Why it's right: Some of what was weighed stays stuck to the dish. Buffer that is going into the flask regardless can collect it on the way, so the mass on the balance becomes the mass in the flask.
- A: Tare is a setting held by the balance for the object on its pan. It is not something a liquid can carry away.
- B: Cleaning the boat is a side effect and not the reason. The point is where the rinsed powder ends up, not how the boat looks.
- C: The buffer and the boat are both at room temperature, so nothing is being cooled. Cooling only becomes an issue after the heating step.
Aligned to Handling, Preparation, Storage and Disposal · reading level ~grade 9
The protocol measures 50 mL of buffer into the flask, then a further 10 mL through the weigh boat. How much buffer is in the flask when both steps are finished?
Reviewed- A.50 mL, since the second measure stays in the boat
- B.It depends on how much powder was weighed
- C.60 mL, since both measures end in the flask
- D.10 mL, since the first measure is a rinse
Show the worked solution ▾
Answer: C. 60 mL, since both measures end in the flask
- Step 1: Follow each measure to its destination: The first goes straight in. The second goes through the boat and then in.
- Step 2: Add them: Both arrive, so the flask holds the sum.
Why it's right: Both measures finish in the same flask. The second one takes a detour through the dish to collect what was left there, and it still ends up in the flask, so the totals add.
- A: The second measure is deliberately tipped out of the boat and into the flask, so it does not stay behind.
- B: Buffer volume is set by the two measures, which are the same whatever the balance read. The mass of powder does not change either one.
- D: The first measure goes into the flask directly and is never used as a rinse. That role belongs to the second one.
Aligned to Handling, Preparation, Storage and Disposal · reading level ~grade 9
- A student finishing the weighing steps checks the boat for powder before setting it down, because that powder is already part of the recipe.
Fill these in as you work through the lesson.
- Tare (zero the balance with the container on it):
- Weigh boat (the small dish the powder sits in):
- Tolerance (the plus or minus band around a target):
- Quantitative transfer (getting all of it into the flask, not most):
Put the boat on the balance, press so the reading returns to zero, then add powder until it reads grams. The tolerance is plus or minus grams, and the second buffer addition goes through the before it goes into the flask.
- Explain what the balance is actually reading before you press tare.
- Work out the lowest and highest masses that still count as acceptable.
- Say why the second measure of buffer takes a detour through the weigh boat.
The boat goes on and the display shows its mass. Press tare and it shows zero even though the boat has not moved. Now every gram you see is agarose. Stop anywhere in the acceptable band, fold the boat, tip the powder into the flask, and tap it to move what clings to the corners.