2.3

Question 1

2.3 Identification of an unknown carbonate

Answer 1

• prepare a solution of an unknown Group 1 carbonate, X2CO3.
• carry out a titration of this solution with 0.100 mol dm–3 hydrochloric acid.
• identify X in the formula X2CO3

Question 2

hazard Solid metal carbonate, X2CO3.

Answer 2

Irritant

Question 3

hazard Hydrochloric acid, HCl(aq), of concentration 0.100 mol dm–3.

Answer 3

Irritant

Question 4

hazard Methyl orange as indicator.

Answer 4

Minimal hazard

Question 5

Equipment

Answer 5

•	Safety spectacles
•	Burette
•	White tile
•	Pipette (25.0 cm3) and filler
•	Clamp stand, with boss and clamp (for supporting the burette)
•	Filter funnel
•	Measuring cylinder (100 cm3)
•	Glass rod
•	Dropping pipette
•	Volumetric flask and stopper (250 cm3)
•	Wash bottle containing distilled (or deionised) water (about 300 cm3 will be required)
•	Two conical flasks (250 cm3)
•	Glass beakers (250 cm3 and 100 cm3)
You will also need access to the following:
•	Top-pan balance weighing to 0.01 g
•	Dropping bottle containing methyl orange indicator

Question 6

Procedure

Answer 6

1. Weigh the bottle provided, containing X2CO3.
2. Tip the solid into a 250 cm3 beaker and re-weigh the empty weighing bottle.
3. Dissolve the solid carefully in about 100 cm3 of distilled water.
4. Transfer all of this solution into a 250 cm3 volumetric flask.
5. Make the solution up to 250 cm3 using distilled water.
6. Invert the volumetric flask several times before use, to mix the solution thoroughly.
7. Using a pipette and filler, transfer 25.0 cm3 of this solution into a conical flask.
8. Add 3–4 drops of methyl orange indicator.
9. Fill the burette with 0.100 mol dm–3 hydrochloric acid.
10. Record all burette readings to 0.05 cm3.
11. Carry out a trial titration.
12. The colour change at the end-point is from yellow to orange.
13. Now carry out the titration accurately and obtain two concordant values for the titre.

Question 7

Analysis

Answer 7

In all questions show your working.
The equation below represents the reaction that you carried out.
X2CO3(aq) + 2HCl(aq) 2XCl(aq) + H2O(l) + CO2(g)

1. Calculate the amount, in moles, of HCl in your mean titre.
2. Using the equation above, calculate the amount, in moles, of X2CO3 used in the titration.
3. Calculate the amount, in moles, of X2CO3 present in the 250 cm3 solution that you prepared.
4. Calculate the molar mass, in g mol–1, of X2CO3.
5. Calculate the relative atomic mass of X.
6. Deduce the identity of X in the formula X2CO3.

Question 8

1. The balance has a maximum error of ±0.005 g in each reading. Calculate the percentage error in your mass of X2CO3.

Answer 8

1. Calculation assuming the mass has been determined from two mass readings:
   % error = 0.005x2/2.1 × 100 = 0.48%

Question 9

2. The burette has a maximum uncertainty of ±0.05 cm3 in each reading. Calculate the percentage error in your first accurate titre.

Answer 9

2. This will depend on student results but using specimen results % error = 0.005x2/22.6 × 100 = 0.44%

Question 10

3. The volumetric flask has a maximum error of ±0.3 cm3 and the pipette has a maximum uncertainty of ±0.04 cm3. Which of these gives the larger percentage error?

Answer 10

3. % error for volumetric flask =0.3/250 × 100 = 0.12%
   % error for pipette = 0.04/25 × 100 = 0.16%
   Therefore, the pipette has the greater % uncertainty.