concentrations, dilutions and dosages

Question 1

What is meant by an amount of a substance and what units can be used to describe an amount?

Answer 1

There are two ways to measure the amount of a substance: count individual bits of it or determine its mass.

Counting to measure amount: count the particles (ions, atoms, molecules).

Molar mass: For example, how many grams are there in one
mole of glucose (C6H12O6)?

Question 2

Define a mole and give the equation to calculate it

Answer 2

The Mole (mol)- 1 mole = amount of substance that has 6x1023 particles. 1 mole = as many particles as there are in 12g of carbon. So, 1 mole of any substance is equal to its atomic or molecular weight in grams. That means, we can find the number of moles by dividing the mass of a substance by its molecular or atomic mass.

No. of moles = mass (g)/Mr (g/mol)

Question 3

How can the mass of a substance be calculated from the moles

Answer 3

E.g. how many grams are there in one mole of glucose (C6H12O6)?

First, find the relative atomic masses of C, H and O. Then use the ratios of the molecular formula: (6x12) + (12x1) + (6x16) = 180g/mol

So, 180g of glucose contains 1 mol of glucose or 6x1023 molecules, or, 1 mol of glucose has a mass of 180g or, the ‘molar mass’ of glucose is 180g/mol.

Question 4

What is meant by concentration?

Answer 4

The amount of solute dissolved in a given amount of solvent. Concentration can be expressed in a number of ways: molarity, parts, weight in volume, volume in volume, weight in weight, % solutions.

Question 5

How do you convert concentration to molarity

Answer 5

Measured in moles per litre (1 cubic decimetre or 1000cm3). The common unit is molar (M).

Divide the number of moles by the volume in litres (or dm3).

To find the number of moles in a solution, multiply the concentration by the volume in L.

To find the volume of a solution, divide the moles by the concentration in mol L-1.

Question 6

How can concentration be expressed in parts?

Answer 6

The ‘parts’ refer to the relationship between the solvent and the solute. Usually ppm (parts per million) or ppb (parts per billion). One part per million is equivalent to- 1 mg in 1 L (dm3), 1 μg in 1 ml. Not used commonly.

Question 7

How can concentration be expressed as a percentage solution?

Answer 7

Percentage concentrations describe w/v, w/w, v/v preparations. In veterinary preparations w/v preparations are most commonly used: Percent w/v means the parts of the solute per 100 parts of the solution, or number of grams in sufficient volume to make up to 100 ml of solvent.

So a 100% solution contains 100g per 100 ml of solution.

So a 5% solution contains 5g per 100 ml of solution.

So a 0.9% saline solution contains 0.9g NaCl per 100ml of water

Question 8

What are the fundamental units of measurement and how can they be converted between commonly used lab units?

Answer 8

• Length- metre (m)
• Mass- gram (g)
• Volume- litre (L, l (dm3))

Question 9

Describe a simple dilution and define the dilution factor

Answer 9

Simple dilutions involve combining a volume of a solution of known
concentration (stock solution) with an appropriate volume of solvent.

The dilution factor is the total
number of unit volumes in which your material is dissolved.

For example, a 1 in 5 dilution means combining 1 unit volume of the solution with 4 volumes of the solvent. Not the same as a ratio- means 1 in a total of 5. Dilution factor = 5

Question 10

Describe a dilution series

Answer 10

Serial dilutions are a convenient way to prepare a wide range of
concentrations or amplify the dilution factor quickly and accurately and they require small volumes. For example: 1mol/l ÷ 100 = 10mmol/l

10mmol/l ÷ 1000 = 10µmol/l

Question 11

Describe how dosages are calculated

Answer 11

Dose = the quantity of drug to be delivered to a patient.

Calculate the dose of the active ingredient per kg of bodyweight of the animal. (e.g. 0.1mg/kg x 350kg horse=35mg).

Dose of active ingredient divided by the concentration of the active ingredient in the drug to calculate the volume (e.g. 35mg÷20mg/ml concentration=1.75ml)

Question 12

Describe how infusion rates can be calculated

Answer 12

The rate to run intravenous fluids can be determined from: the volume of fluid needed, time of administration, number by the drops per millilitre delivered by the giving set.

To calculate the drip rate:

Volume of fluid required/time x drop factor (the number of drops in a ml) to calculate the number of drops per minute. Divide this by 60 to get the number of drops per second and divide this by 1 to get the number of drops per second.