In this following experiment, we are aiming to determine the relative atomic mass of magnesium, using a process known as ‘back titration’. The basic outlines of this experiment include a strip of magnesium being allowed to reach with excess hydrochloric acid. The excess acid is then determined by titration with a standard alkali (Sodium Hydroxide) and hence the moles of acid that reacted with the magnesium is found by difference. This can be used to determine the number of mole of magnesium used (explained more thoroughly in analysis section). The relevant equation is:

2H+ (aq) + Mg (s) –> Mg2+ (aq) + H2 (g)

Safety Safeguards

Chemical or Apparatus



Hydrochloric acid


Avoid contact with skin, handle properly, wash off any spileges, wear attention protection.

Sodium Hydroxide


See previously mentioned

Methyl lemon indicator

Unsightly stains

Handle along with care, avoid contact with epidermis


* 250cm3 volumetric flask

2. 25cm3 pipette

* Pipette for filler injections

* Burette

* Burette grip with stand

* 250cm3 conical display

* Light tile


* Clear magnesium ribbon

* Standard 1 . 0M hydrochloric acid answer

* Indicator (screened methyl orange)

Variables and Fair Tests

A reasonable test must be ensured always, in any experiment, in order to keep the outcomes as accurate as possible so that appropriate conclusions can be drawn. The main way that will I hope to achieve this particular is by repeating each of my results a more 2 times so that a good average can be used and any anomalous effects can be spotted prior to they may be taken as authentic ones. Along with this I must consider how correct I want my leads to be.

Repaired Variables

Same volume of hydrochloric acid solution (now reacted with the particular magnesium): As we’ re also attempting to determine the particular relative atomic mass associated with magnesium, then the acid solution it is dissolved inside should be kept constant throughout the experiment, otherwise even more than one variable would certainly occur. This would therefore mean our results show nothing of value, in addition to as such determining the extra acid by titration can be of no use.

Same drops associated with Indicator: The quantity of drops associated with indicator used (methyl orange) must be kept constant throughout. The number regarding drops should actually end up being one or two, normally distinguishing the colour alter through the titration would show difficult, whereby the probability of errors would boost. A white tile will be also used to aid us in the help of distinguishing the coloring change.

Same exact Apparatus: It is the necessity that all apparatus is kept the same throughout the experiment. Diverse apparatus differ in their particular accuracy. For example, 50cm3 beaker would be more accurate than that of a 250cm3. Also, all equipment should be washed carefully before and after each experiment, to be able to avoid any possible contaminants.


Same concentration regarding acid and alkali: Inside a solution whereby typically the concentration is high, in that case the particles that consist are closely packed. This specific results in more regular, successful collisions. ****BRIGGS*** In no way had time to finish this part off.


The volume of standard alkali (sodium hydroxide): The aim of the analysis is to determine the relative atomic mass of magnesium (mg) by back titration, and therefore, this can be the only diverse factor in this test. For our experiments to be accurate, we expect just about all results to be concordant, i. e. very comparable, within a +- 0. 1cm3 range. Results beyond or below this could be regarded as anomalous, and therefore will probably be ignored.

Causes of Error

The magnesium may not become clean. It could in elements, potentially be magnesium o2, and also, magnesium comes with a tendency of tarnishing. To prevent this, we can use emery cloth, and wipe the magnesium completely.

The needed mass of magnesium necessary is 0. 3 gr. Weighing this out simply by trial and error would certainly take us too very much time, not to mention how inaccurate it is. So, to overcome this specific problem, simple algebra is used. We would consider 1m of magnesium, consider this out (assume this specific value is K), plus substitute this into the simple equation. This getting.

(0. three or more / k) x (? / 100 )

Correct use in addition to handling of apparatus is also another key factor. With regard to good experimental techniques, i actually. e. handling, setting back up and taking advantage of apparatus, we ought to always:

* Keep the burette vertically

* Ensure there is no air gap beneath the tap

* Rinse out apparatus with distilled water, and then with all the answer being used

* Remove the direct when titrating

* Ensuring the worth of the pipette is read at eye level

* Movements during transferring ought to be retained at a minimum

* Allowing this to drain under gravity

* Any time empty touching the finish on the surface of the solution to leave typically the correct amount of water inside the pipette

* Constantly swirl the conical flask whilst working in the perfect solution is from typically the burette.

5. Towards the end, add in drops to avoid incorporating an excess solution

Repeating the experiment is usually also required. Two en accord titres are needed, if not necessarily, then repeats are taken.


It is usually essential to remember that the particular method integrates ALL options off error (as stated above), to increase precision. The method used is just a procedure, hence is kept simple, and one must recognize of which our company is constantly keeping actually fair.

1. Consider accurately about zero. 3g of the magnesium ribbon.

a couple of. Pipette 50. 0cm3 regarding the 1. 0M hydrochloric acid into a 250cm3 volumetric flask.

3. Break the magnesium ribbon into small (about 1 cm) pieces in addition to carefully drop it to the flask.

four. Allow the magnesium in order to react completely with typically the acid, and then make up in order to 250cm3 of solution along with distilled water.

5. Pipette out twenty-five. 0cm3 of this remedy into a conical flask. Place this on the white tile.

6. Titrate the excessive acid in the answer with standard 0. 100M sodium hydroxide, using a new few drops of processed through security methyl orange indicator.

7. Observe in addition to record all results



Mass of magnesium ribbon (rough) = 0. 32g

Mass of magnesium ribbon (accurate) = a few. 154g


Burette – standard (0. 1< ) sodium hydroxide solution

Pipette – unfamiliar hydrochloric acid solution






Final vol. (cm3)

22. 1

46. 3

24. 3

48. 5

Initial vol. (cm3)


22. 1


24. 3

Titre (cm3)

22. 1

24. 2

24. 3

24. 2

Average tire (cm3)

24. 2

Hence 24. 2cm3 of standard 0. 1M sodium hydroxide solution reacted completely with 25. cm3 of unknown hydrochloric acid solution.


The reactions which were undergone during the reaction were between magnesium (mg) with the hydrochloric acid, and the sodium hydroxide with the magnesium chloride solution. The equations (both word and symbol) are shown below:

Magnesium + Hydrochloric acid – > Magnesium Chloride + Hydrogen

Mg + 2HCl – > MgCl2 + H2

From the results, we are now capable to calculate the number of moles of excess acid in 250cm3 of the solution. To do this, we need to be in a position to calculate the quantity of moles in salt hydroxide, using the next equation:

Number of moles = quantity (in dm3) x molarity

Substituting the particular values gives us:

Amount of moles sama dengan (25 / 1000) times 0. just one

= 2 . 5 x 10-3 skin moles

Therefore, for calculating typically the number of moles excessively hydrochloric acid, we increase our answer (number regarding moles in NaOH) by simply 10. This gives us all 0. 025 moles.