Weight & MeasurementsArya Mehr

Question 4: Part A

You have a prescription balance with a sensitivity requirement (SR) of 1mg. What is the percentage error when weighing 30mg?

Equation to know/use:

Plug the two values into the equation

= 3.3% Error

Question 4: Part b

What is the minimum weighable quantity (MWQ) if the sensitivity requirement is 1mg and the desired accuracy is 99%?

Equation to know/use:

Plug the two values into the equation

= 100mg

If no desired accuracy is provided… Assume 95% accuracy (5% error)

Question 5: Part A

You are using a prescription balance with a SR of 6mg. Assume a desired accuracy of 95%. What is the MWQ?

Fill the following prescription with little wastage: DHEA 20mg Lactose qs 400mg 50 capsules

Equation to know/use:

Plug the two values into the equation

= 120mg

Question 5: Part B

How much DHEA will you use?

Fill the following prescription with little wastage: DHEA 20mg Lactose qs 400mg 50 capsules

Helpful Tip: Always make 10% more capsules than you need you require

(

Multiply number of capsules (with additional 10%) by amount of DHEA needed.

Since 1100mg is above MWQ we can measure it accurately

Question 5: Part C

How much lactose will you use?

Fill the following prescription with little wastage: DHEA 20mg Lactose qs 400mg 50 capsules

What in the world is qs?• “qs” is shorthand for the Latin term “Quantum satis” • Quantum satis: amount which is needed

Try to reason through this problem… Each capsule has to weigh 400mg and you need 20mg of

DHEA in each capsule This means that you would need 380mg of Lactose to fill the

rest of the capsules

Since you need 380mg of lactose in each capsule and you have 55 capsules…

Question 6: Part A

You are using a prescription balance with a SR of 5mg. Assume a desired accuracy of 95%.

Fill the following prescription with little wastage: Atropine sulfate 2.5mg Lactose qs 400mg 10 capsules

Equation to know/use:

Plug the two values into the equation

= 100mg

You can only measure up to 100mg unless you want to be more than 5% off!

Question 6: Part B

How much Atropine will you use?

Fill the following prescription with little wastage: Atropine sulfate 2.5mg Lactose qs 400mg 10 capsules

Helpful Tip: Always make 10% more capsules than you need you require

(

Multiply number of capsules (11) by amount of Atropine needed (2.5mg per capsule).

Question 6: Part B

But wait! Remember that our MWQ (minimum

weighable quantity) was 100mg You can not weight compounds less than 100mg

unless you are willing to accept an error of more than 5% (it will NOT be accurate)

Since 27.5mg is not above 100mg MWQ we need to multiply!

Try to Get Above 100mg MWQNO

YES

Question 6: Part B

So now you have 110mg of Atropine which is four times the amount that you need.

REMEMBER you can NOT weight anything less than 100mg since it is the MWQ

We must dilute the atropine with lactose so that 1/4th of the total mixture will be above 100mg MWQ

1 part atropine (27.5mg)3 parts lactose (3 x 27.5mg)

4 parts mixture (110mg, which is above MWQ)

Question 6: Part B

But wait! That you have 4 parts of atropine sulfate

(110mg) You have to then multiply everything by 4

1 part atropine (27.5mg) x 43 parts lactose (3 x 27.5mg) x 4

4 parts mixture (440mg, which is above MWQ)

(330mg lactose used)

Question 6: Part C

How much lactose did you use including initial dilution?

Fill the following prescription with little wastage: Atropine sulfate 2.5mg Lactose qs 400mg 10 capsules

mg x 11 capsules = 4,400mg

Since you have 110mg of Atropine and Lactose you must subtract from the number (remember it is qs)

mg – 110mg = 4,290mg

However… we used 330mg during the initial dilution (previous slide) so we must add that to the total

mg + 330mg = 4,620mg lactose