Finding Out How Much Acid There Is In A Solution

Finding Out How Much Acid There Is In A Solution

Aim

The aim of the investigation is to find out the concentration of a sample of sulphuric (VI) acid. The solution is thought to have a concentration between 0.05 and 0.15 mol dm .

To find this out I will be using the method of titration. This is a quantitative technique that involves reacting a substance of known concentration and volume, with the substance that we want to find out the concentration of. In this particular case I will use anhydrous sodium carbonate as the known substance and sulphuric acid is the unknown.

Titration is a suitable experiment to do as, from the results I will be easily able to gather results and find out the unknown concentration.

Background

The equation for this reaction is:

H SO (aq) + Na CO (aq) CO (g) + H O (l) + Na SO (s)

Sulphuric Acid + Sodium Carbonate Carbon Dioxide + Water + Sodium Sulphate

The ratios of reacting sulphuric acid and sodium carbonate are 1:1. As a known concentration of sodium carbonate is need, I have decided to have this concentration as 0.10 mol dm , I have chosen this value as it is the midpoint of the estimated concentration of sulphuric acid (0.05 - 0.15 mol dm ). I think this is suitable because the ratio is 1:1 so the midpoint will be near to estimated one.

Making Up Solution of Known Concentration

Now I will work out how much to use for the sodium carbonate solution.

First I need to work out the molecular mass of the sodium carbonate I will work this out by adding up the atomic masses:

2 x Na = 46.0

1 x C = 12.0

3 x O = 48.0

So I would need to use 106.00g is a litre of water for a 1 mol dm solution. So as I want a 0.10 mol dm solution I will need to divide 106.0 by 100 = 10.60g. Therefore I will need to put 10.60g in 1000.00cm for a 0.10 mol dm solution.

I have now found my standard solution. However, I only need 250.00cm of solution so I will divide 10.60g by 4 = 2.65g. Therefore in 250.00cm of distilled water I will need to add 2.65g of sodium carbonate to give me a 0.10 mol dm solution. However, I will be weighing by difference so I may add a little less, but as long as I record this then I shall be able to use those values when finding my final answer.

Indicator

As I am using a strong acid (sulphuric acid) and a weak alkali (sodium carbonate) the resource given to me says methyl orange should be used as the indicator. As this is a neutralisation reaction, the indicator will tell me when the solution is neutral. The indicator will change colour when this happens. In a solution becoming less acidic (i.e. towards neutral), methyl orange moves from red through orange and to yellow. So as soon as a see that the yellow is turning darker, into orange, then when it reaches red will be my end point. I will only use about 2 -4 drops in the solution because not very much is needed, I am only looking for a colour change. Methyl orange's structure is simple enough to be able to see what is happening as it loses and gains hydrogen ions:

This result is shown in the following diagram:

It is obvious in the diagram below that phenolphthalein would be completely useless. However, methyl orange starts to change from yellow towards orange very close to the equivalence point. Therefore I am looking for an endpoint of when the solution goes from a yellow to a red. From previous experiments in using methyl orange, I know that this red appears slightly pink/red. When the solution turns this colour, I have reached my end point.

So the indicator, which should be chosen, is the one that changes colour on the steep bit of the curve.

Risk Assessment

Throughout the experiment I will wear protective clothing (i.e. lab coat), gloves and goggles. As I am using glassware, if any is broken I will sweep it up and put it in the broken glass bin. If I use the Bunsen burner I will use a heat mat to protect surface. As I have long hair I will tie my hair back to prevent it from getting in the way and I will also make sure that all chairs are tucked under the tables in the lab I am working in, as otherwise they could be a trip hazard.

Sodium Carbonate (Na CO ):

Corrosive causes burns and severe irritation. In case of contact with eyes rinse immediately with plenty of water for at least ten minutes. If spilt on clothing, remove contaminated clothing. Harmful if swallowed, inhaled or absorbed onto skin. If swallowed drink plenty of water and do not induce vomiting. If spilt on skin, soak with water. If any of these happen seek medical advice as soon as possible.

Sulphuric Acid (H SO ):

Very corrosive, causes severe burns. As I intend to make a solution less than 0.5 mol dm bottle should be labelled irritant. If swallowed, wash out mouth and give glass r two of water, do not induce vomiting. If splashed in the eye, flood eye with water for ten minutes. In both cases seek medical attention. If spilt on skin or clothes, remove contaminated clothing, wipe as much as possible of the skin and soak in water. If the skin is blistered seek medical attention. If the acid is spilt in a lab cover with a mineral absorbent and scoop into bucket, add anhydrous sodium carbonate over mixture to neutralise and rinse spilled area several times with water.

The following are products of the reaction that also need to be risk assessed:

Carbon Dioxide (CO ):

Not very dangerous, but in large quantities do not breathe in. If person is exposed to large amount, place them in fresh air for a period of time. It is also