2.2 Relative mass
OCR Chemistry A Teacher and Technician notes
Determining the relative atomic mass
of a metal
Specification references
• 1.1.1 c) • 1.2.1 a) b) c) d) e) j)
• 1.1.2 a) b) c) • 1.2.2 a) k)
• 1.1.3 a) b) c) • 2.1.3 e) (i) (ii) f) h)
• 1.1.4 a) c) d) • This practical includes techniques and
skills relevant for PAG 1
Aims
• In this practical the students collect hydrogen gas when it is bubbled through water in order
to calculate the relative atomic mass of an unknown Group 1 metal, based on the
stoichiometry of an equation they are given.
• The students will use a measuring cylinder to measure the volume of gas evolved in this
experiment, rather than a burette or gas syringe. This means that they will not be able to
obtain very precise readings. The students should be encouraged to consider the sources of
imprecision in the method. In the follow up sheet, students consider two different methods for
finding the relative atomic mass of a metal and hence should develop skills in evaluating
practical techniques. They should be able to decide which method is best, considering all the
evidence.
Learning outcomes
After completing the practical students should be able to:
• carry out calculations using stoichiometric relationships
• measure the volume of gas collected over water using a delivery tube and measuring
cylinder
• determine the relative atomic mass of a metal by using experimental data.
Teacher notes
• The experiment should take approximately 30 minutes.
Example data
Mass of lithium used is 0.10 g
3
Volume of hydrogen collected is 150 cm
© Oxford University Press 2015 www.oxfordsecondary.co.uk/acknowledgements
This resource sheet may have been changed from the original 1
, 2.2 Relative mass
OCR Chemistry A Teacher and Technician notes
Answers for method sheet
volume
1 Moles of hydrogen =
24800
150
= (1 mark)
24800
–3
= 6.05 × 10 (1 mark)
–3
2 Moles metal = 6.05 × 10 ×2 (1 mark)
= 0.0121 (1 mark)
mass
3 Molar mass of metal =
moles
0.10
= (1 mark)
0.0121
= 8.3 (1 mark)
The metal is therefore lithium. (1 mark)
4 Step 4 to prevent oxidation of the Group 1 metal in air (although this would be minimal). (1 mark)
Step 5 to prevent hydrogen from escaping. (1 mark)
–
5 Metal hydroxide is produced, and hence hydroxide (OH ) ions in the solution increase in
concentration. (1 mark)
+
Concentration of hydrogen (H ) ions does not increase, since the hydrogen bubbles off
as a gas. (1 mark)
The pH goes up from pH 7 (neutral, pH of water) (1 mark)
It is therefore likely to be greater than pH 8. (1 mark)
© Oxford University Press 2015 www.oxfordsecondary.co.uk/acknowledgements
This resource sheet may have been changed from the original 2
OCR Chemistry A Teacher and Technician notes
Determining the relative atomic mass
of a metal
Specification references
• 1.1.1 c) • 1.2.1 a) b) c) d) e) j)
• 1.1.2 a) b) c) • 1.2.2 a) k)
• 1.1.3 a) b) c) • 2.1.3 e) (i) (ii) f) h)
• 1.1.4 a) c) d) • This practical includes techniques and
skills relevant for PAG 1
Aims
• In this practical the students collect hydrogen gas when it is bubbled through water in order
to calculate the relative atomic mass of an unknown Group 1 metal, based on the
stoichiometry of an equation they are given.
• The students will use a measuring cylinder to measure the volume of gas evolved in this
experiment, rather than a burette or gas syringe. This means that they will not be able to
obtain very precise readings. The students should be encouraged to consider the sources of
imprecision in the method. In the follow up sheet, students consider two different methods for
finding the relative atomic mass of a metal and hence should develop skills in evaluating
practical techniques. They should be able to decide which method is best, considering all the
evidence.
Learning outcomes
After completing the practical students should be able to:
• carry out calculations using stoichiometric relationships
• measure the volume of gas collected over water using a delivery tube and measuring
cylinder
• determine the relative atomic mass of a metal by using experimental data.
Teacher notes
• The experiment should take approximately 30 minutes.
Example data
Mass of lithium used is 0.10 g
3
Volume of hydrogen collected is 150 cm
© Oxford University Press 2015 www.oxfordsecondary.co.uk/acknowledgements
This resource sheet may have been changed from the original 1
, 2.2 Relative mass
OCR Chemistry A Teacher and Technician notes
Answers for method sheet
volume
1 Moles of hydrogen =
24800
150
= (1 mark)
24800
–3
= 6.05 × 10 (1 mark)
–3
2 Moles metal = 6.05 × 10 ×2 (1 mark)
= 0.0121 (1 mark)
mass
3 Molar mass of metal =
moles
0.10
= (1 mark)
0.0121
= 8.3 (1 mark)
The metal is therefore lithium. (1 mark)
4 Step 4 to prevent oxidation of the Group 1 metal in air (although this would be minimal). (1 mark)
Step 5 to prevent hydrogen from escaping. (1 mark)
–
5 Metal hydroxide is produced, and hence hydroxide (OH ) ions in the solution increase in
concentration. (1 mark)
+
Concentration of hydrogen (H ) ions does not increase, since the hydrogen bubbles off
as a gas. (1 mark)
The pH goes up from pH 7 (neutral, pH of water) (1 mark)
It is therefore likely to be greater than pH 8. (1 mark)
© Oxford University Press 2015 www.oxfordsecondary.co.uk/acknowledgements
This resource sheet may have been changed from the original 2
