Module 1 Development of Practical Skills in Chemistry

Question 1

When there is no uncertainty of graduated apparatus, how can it be calculated?

Answer 1

Half of the smallest graduation

Question 2

When no uncertainty is provided for digital apparatus, how can it be calculated?

Answer 2

It is assumed to be the resolution of the apparatus e.g., a 2 decimal place weighing balance has an uncertainty of +/- 0.01 in each measurement

Question 3

Formula for percentage uncertainty

Answer 3

Percentage uncertainty = uncertainty / quantity measured x 100

Question 4

A mass balance was used twice, what must we do to the uncertainty?

Answer 4

Multiply by 2

Question 5

List the equipment where uncertainty must be multiplied by two

Answer 5

1. Thermometer (when calculating temperature change)
2. Burette
3. Mass balance

Question 6

List ways in which percentage error may be reduced

Answer 6

1. Increase amount measured
2. Increase precision by using a two decimal place weighing balance

Question 7

Describe the trends in atomic radii across a period

Answer 7

• Protons added to nucleus so nuclear charge increases
• Electrons added to same shell
• Nuclear attraction on outer shell electrons increases across a period
• Electron shells drawn in by nucleus

Atomic radii decreases across period

Question 8

Explain trends in atomic radii down a group

Answer 8

• Number of shells increases
• Outer electron is added to new shell further away from nucleus
• Shielding effect by inner shells increases down a group
• Increase in distance far outweights increase in nuclear charge
• Nuclear attraction decreases down a group

Atomic radii increases down a group

Question 9

Which acronym can be used when describing trends in atomic radii

Answer 9

Now = nuclear charge
Do = distance from nucleus
Something = shells (electron repulsion from inner shells)
Amazing = attraction (nuclear)

Question 10

Describe melting and boiling points between group 1 + 4

Answer 10

Increases gradually

Question 11

Describe melting and boiling points between group 4 + 5

Answer 11

Decreases sharply (goes from metallic to simple covalent)

Question 12

Describe melting and boiling points between group 5 + 8

Answer 12

Comparatively low