Practical 1

Question 1

Introduction

Answer 1

Analytical skills essential for sampling and measure accurately substances. Quantitative analysis.

Question 2

Aims

Answer 2

correct usage of scientific weighing and measuring
Accuracy and precision results
1 sample t-test

Question 3

Accuracy definition

Answer 3

It is expressed as the maximum allowable error.

Volumetric flasks and pipettes expected tolerance or ‘limit of error’ and these are quoted at 20 oC (see tables below).

Glass will expand (or contract) at different temperatures.

Notice that the limit of error (mL) increases as volume increases.

However, the limit of error as a percentage decreases as volume increases.

Question 4

The use and care of volumetric glassware

Answer 4

All glassware should therefore be clean. Maintain the quality. The glassware used depends on both the volume measured and accuracy required.

PRECISION GLASSWARE
Volumetric flasks
Volumetric pipettes
Volumetric burettes

INEXACT GLASSWARE
Beakers
Conical flasks
Measuring cylinders

Question 5

Types of balances and weighing procedure

Answer 5

The smaller the amount to be weighed, the greater the accuracy required.

If only a rough weight is required a top-loading (or a top-pan) balance is used
Analytical balances are used for accurate work

Question 6

Pipettes

Answer 6

Transfer / deliver aliquots (accurately measured volumes) of liquids from one container to another

• Volumetric or transfer pipettes to deliver a single volume, e.g., 5, 10, and 25 mL of a liquid. A small amount of the liquid remains in the tip when it is emptied and it must not be blown out.
• Measuring or graduated pipettes capable of delivering variable quantities of a liquid.

It must be noted whether these pipettes have a scale relating to a delivery, or a blow-out, volume.

……..Pipettes with a blow-out volume scale use the small amount of the liquid remaining in the tip when it is emptied must be also be used to obtain the correct volume.

……..If the pipette has DELIVERY, the small amount of the liquid that remains in the tip when it is emptied must NOT be blown out.

Question 7

Volumetric flasks

Answer 7

Standard solutions of accurately known concentrations.

1. First accurately weigh the desired amount using an appropriate balance and a weighing boat.
2. Then transfer the material to a beaker. Rinse the weighing boat at least twice with small amounts of solvent.
3. Dissolve the material in a beaker, and then transfer this solution to the flask, via a funnel.
4. Rinse the beaker, at least three times with small portions of solvent transferring the rinsing slowly into the funnel / flask, avoiding any spillages.
5. After all the solution has been transferred into the flask, the funnel should also be rinsed at least three times with small portions of solvent, and these rinsings transferred into the flask.

Ensure that during the above procedures the liquid level does not rise above the etched mark on the flask.

Question 8

Precision definition

Answer 8

It is expressed by the uncertainty of a reading.

To attain the degree of precision required, a quantity should be “accurately measured”.

The apparatus chosen must be suitable to the desired volume required.

Question 9

Concentration units

Answer 9

Molarity, M: number of moles of the solute per litre

weight/volume basis % (w/v) – the mass of the solute / volume of the solution

volume / volume basis % (v/v) – the volume of the solute / the volume of the solution

Question 10

Dilutions

Answer 10

C1 x V1 = C2 x V2

C1 = concentration of stock solution in the given units 
V1 = volume of the stock solution taken for dilution 
C2  = concentration of the diluted solution in the same units
V2 = volume the diluted solution

The moles of the solute / analyte taken for dilution from the stock solution are the same as the moles in the final diluted solution.

Question 11

Reporting results

Answer 11

Arithmetic mean or average
Accuracy
Precision: coefficient of variation, also called relative standard deviation
Population and standard deviation

Question 12

Weighing

Answer 12

Solids:

1. Accurately weigh approximately 1 g of the white powder.
2. Record your data (to 3 decimal places).
3. Repeat steps 1 and 2 three times (you should have weighted out the powered 4 times.
4. Calculate the mean, standard deviation and % coefficient of variation (CV) of your data.
5. Reflect on the accuracy and precision of your weighing technique and record suitable comments in your lab notebook

Liquids:

1. In turn weight each beaker and then accurately pipette 10 mL of ultra-pure water (UPW) into it. Reweigh the beaker + UPW. Record all your data in tabular form in your lab notebook.
2. Calculate the mean and standard deviation and %CV of the 5 masses of UPW.
3. Reflect on the accuracy and precision of your pipetting technique and record suitable comments in your lab notebook.