Question

A prescription balance has a sensitivity requirement of 6.5 mg. Explain how you would weigh 20 mg of a substance with an error not greater than 2%.

Weigh 400 mg drug; Use 400 mg as aliquot

Answer 1

To weigh 20 mg of a substance with an error not greater than 2%, you can use the concept of aliquots and calculate the maximum allowable error. The maximum allowable error for weighing 20 mg of the substance is 0.325 mg.

Step-by-step explanation:

To weigh 20 mg of a substance with an error not greater than 2% using a prescription balance with a sensitivity requirement of 6.5 mg, you can use the concept of aliquots. An aliquot is a measured portion of a larger sample. In this case, you can weigh a larger amount of the substance, such as 400 mg, and use it as an aliquot. Then, you can use the known weight of the aliquot to calculate the weight of the desired 20 mg.

Let's say you weigh 400 mg of the substance. Now, divide the sensitivity requirement (6.5 mg) by the aliquot weight (400 mg) and multiply it by the desired weight (20 mg). This will give you the maximum allowable error for the 20 mg of substance.

Using the given values, the calculation would be (6.5 mg / 400 mg) x (20 mg) = 0.325 mg. Therefore, the error for weighing 20 mg of the substance should not be greater than 0.325 mg.