Accuracy & Precision & Unit Conversions

Question 1

What is accuracy?

Answer 1

Accuracy is the closeness of the results to the true value.

Question 2

What is precision?

Answer 2

Precision is the closeness of the replicate results to each other.

Question 3

What is a primary standard?

Answer 3

A primary standard is a compound which is easily purified, dried and stable, and is used to standardise solutions.

Question 4

What is a secondary standard?

Answer 4

A secondary standard is a compound which is easily dissolved to form stable solutions, and is standardised against a primary standard.

Question 5

What is a certified reference material?

Answer 5

A certified reference material is a substance used for calibration, whose values were certified by a technically valid procedure from several well-established laboratories, using different techniques, and is accompanied by a certification/ document.

Question 6

When would a certified reference material be used in analytical science?

Answer 6

calibrate instruments,
validate analytical methods,
quality control,
transfer results between labs

Question 7

What is absolute analysis?

Answer 7

Absolute analysis involves chemical reactions that go to stoichiometric completion.

Question 8

What is comparative analysis?

Answer 8

Comparative analysis involves preparing standards and comparing a measurement of a(n) (unknown) sample against known standards (of known concentrations).

Question 9

Give an example of absolute analytical measurement.

Answer 9

pH, conductance

Question 10

Give an example of comparative analytical measurement.

Answer 10

pH, absorbance

Question 11

What are the characteristics of a certified reference material?

Answer 11

A certified reference material has properties which have been certified by a technically valid procedure, and is accompanied by certification.

Question 12

What are the requirements of a primary standard?

Answer 12

Primary standards should:
- be easy to purify and dry, and maintain in a pure state
- not change in the air during weighing (i.e. absorb moisture or be photochemically unstable)
- have a high molar mass to minimise weighing errors
- be at least 99.98% (w/w) pure.
- react instantaneously with the solute, according to a stoichiometric ratio
- be soluble in the conditions which it will be used

Question 13

What are the requirements of a secondary standard?

Answer 13

Secondary standards can be dissolved to form stable solutions.

Question 14

Give an example of a primary standard.

Answer 14

Potassium iodate, sodium chloride

Question 15

Give an example of a secondary standard.

Answer 15

Sodium thiosulphate, sodium hydroxide

Question 16

What are external standards?

Answer 16

External standards involve preparing a set of standards of pure analyte at different concentrations, and measuring a signal with an instrument, then drawing a calibration curve.

Question 17

Define molarity.

Answer 17

Molarity is the number of moles per litre.

Question 18

Define molality.

Answer 18

Molality is the number of moles per kg.

Question 19

Define normality.

Answer 19

Normality is the number of equivalence per litre.

Question 20

How is precision measured?

Answer 20

Precision is measured with standard deviation.

Question 21

How is accuracy measured?

Answer 21

Accuracy is measured with percentage error.

Question 22

How would a graph that would be obtained when using external standard calibrations look like?

Answer 22

The line would go through the origin

Question 23

How would a graph that would be obtained when using internal standard calibrations look like?

Question 24

How would a graph that would be obtained when using standard additions calibrations look like?