Lecture 3 : Calculations- Importance And Accuracy

Question 0

Why are accurate calculations important?

Answer 0

Want a safe product with an accurate amount
Treat patients as if they were a close relative as serious harm could be done otherwise

Check doctors prescription of dose

Careful methodical working required

Need to relate calculations to practice and visualise

Think through first before calculating

Double check everything

Question 1

What calculations do we need to be familiar with?

Answer 1

Prepare specific strength of solution
Volume of preservatives
Number of parts to two or more components to be mixed when the final desired concentration is known
Paediatric doses
Solid dose formulations like suppositories
Number of tablets to use when making an oral liquid formulation

Question 2

What are the units for weight volume and length?

Answer 2

Kg, L, m

Question 3

What are the common conversions pharmacists need to be able to make?

Answer 3

1L = 1000mL
1kg = 1000g
1g = 1000mg
1mg = 1000μg

Question 4

How do you deal with explaining measurements to patients?

Answer 4

Do not assume they know what you know
Offer a measure e.g. 1 standard teaspoon = 5mL, 1 standard tablespoon = 15mL (3tsp)

A teaspoon at home is not a standard measure unless they are a baking standard

Check that the patient understands

Question 5

If you were asked to prepare 500 mL of a 1:8000 solution of X, how much X is needed?

Answer 5

1:8000 = 1g in 8000mL
500/8000x1 = 0.0625g
= 62.5mg

Question 6

How do you weigh the drug when the smallest amount is less than the minimum weight you can weigh on the scales?

Answer 6

Weight twice the amount (125mg)
Dissolve it in 100mL solution
Use 50mL of this and make it up to 500mL

Question 7

What happens if the drug has low solubility?

Answer 7

May need to dilute in more than 100mL but the concept is the same:

Make it in the same concentration, then use the volume that is required

Question 8

How much Y is needed (in mg) to prepare a 200mL solution containing 3μg/5mg of Y?

Answer 8

Min weight = 100g
Weigh 120mg, dissolve in 200mL and pipettes out 0.2mL which will contain 0.120mg

Or
Weigh 100mg dissolve in 200mL and 0.24mL will contain 0.120g

Question 10

What is %?

Answer 10

How much in 100mL (ALWAYS)

Question 11

what formula is used for most calculations to work out the concentration of preservative?

Answer 11

c1v1=c2v2

Question 12

if the final product was 100mL and you are preserving with a 5% perservative, how much perservative do you need to make a 0.1%w/v MIC?

Answer 12

c1v1=c2v2
5x v1 = 0.1x100
v1 = 10/5
= 5mL

Question 13

what is an alligation?

Answer 13

a method for solving the number of parts of two or more compounds to be mixed when the final desired concentration is known

Question 14

If Mixture A is 400mg/5mL and Mixture B is 100mg/5mL and we need to mix A and B toether to make a 100mL suspension of desired concentration 300mg/5mL, what is the volume of A and B needed?

Answer 14

after doing the cross diagram, add the numbers on the right hand side.

in this example it comes to 300

this means that 300 arts = 100mL suspension
we need 200 parts of A and 100 parts of B (taken from the right side of the diagram)

200/300 x 100 = 66.7mL of A
100/300 x 100 = 33.3mL B

Question 15

how can the alligation be double checked?

Answer 15

by working backwards.
multiply the volume you calculated by the original concentrations of A and B and add the masses together.

in this case it is 6000mg
6000/100x5 = 300mg/5mL as required

Question 16

How do you make 70mL of oral liquid using tablets of Y?

The strength of the final liquid should be 2mg/ml. The tabley strength is 40mg.

Answer 16

you need 2x70 = 140mg of Y
140/40 = 3.5 tablets

You need to use whole tablets so use 4 tablets, giving 160mg of Y
Make the final volume of 160/2 = 80mL then dispense 70mL and throw out the rest.

Question 17

When is a displacement value required?

Answer 17

in all situations where the density of the drug and base are not the same

Question 18

what does the displacement value tell us?

Answer 18

the amount of base displaced by the drug

Question 19

what is a displacement value?

Answer 19

the number of parts by weight of drug which displaces one part by weight of base.

Question 20

in a prescription of 10 x 25 suppositories of medication X of 100mg.

X has a displacement value of 2.7

calulate the amounts of X and the base to prepare suppositories

Answer 20

the mould size is 2g
for 10 suppositories, 10 x 2 = 20g
weight of drug X is 100x10 = 1g

2.7g of X displaces 1g of base
1g displaces 0.37g of X
20-0.37 = 19.63g of base required

Question 21

what are the six ways to minimise caluclation errors?

Answer 21

write out calculations clearly

if transferring data double check what you have written with the reference

write down every step

do not take short cuts

anticipate your answer

double check your claculations. use a different method. work in reverse from answer to starting numbers

Question 22

what are tips for accurate calculations?

Answer 22

always work methodically and write down calculations clearly
check caluclations using a different method where possible
estimate the answer before you start
try to visualise the quantities you are using in the calculations
read the formula carefully.
use correct figures in equations
dont mix units.
think about whether the answer is reasonable.