# Finding Out how much Acid there is in a Solution

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Finding out How Much Acid there is in a Solution

When a metal is extracted from its ore, a waste product is often sulphur dioxide (SO2). This is then converted into sulphuric acid (H2SO4) and sold. To sell sulphuric acid, its accurate concentration must be known. In this investigation, I will use a titration method to attempt to find the accurate concentration of a sample of sulphuric acid thought to have a concentration of between 0.05 and 0.15 mol dm-3. The chemicals which I will use to do this will be solid anhydrous sodium carbonate (Na2Co3) and an indicator.

Finding out How Much Acid there is in a Solution - Plan

Quantities of Chemicals Required

In this investigation, the chemical reaction used will be the neutralisation reaction between sulphuric acid and sodium carbonate:

H2SO4 (aq) + Na2CO3 (aq) → Na2SO4 (aq) + CO2 (g) + H2O (l)

As the sodium carbonate has to be aqueous for this reaction, I will need to make up a solution of sodium carbonate before I begin the titration. I will need to choose an appropriate concentration for this solution and an appropriate volume of it to make up. As there is a 1:1 ratio between the moles of sulphuric acid and sodium sulphate used in this reaction, I should make up a solution of sodium carbonate of similar solution to that of the sulphuric acid. Assuming that the concentration of the sulphuric acid is around 0.1 mol dm-3, this will be the concentration of the sodium carbonate solution I make.

Next I must decide what volume of solution to make up. Firstly I need to decide the volume of solution I will require per titration. This volume will be measures using a pipette and filler, so only set volumes, such as 10cm3, 25cm3 and 50cm3 are available. If I use 10cm3, the percentage error will be too large to produce reliable results, and if I use 50cm3, it is likely that I will need too much acid for a 50cm3 burette. Therefore the best volume of solution to use per titration will be 25cm3, as it allows the acid to have a concentration greater than or less than the assumed 0.1 mol dm-3. To produce reliable results, I will need to complete three titrations with a range of no more than 0.1cm3. Taking into account a rough titration and any anomalous results, I will probably need to do at least six titrations, which will require 150cm3 of sodium carbonate solution. The solution will be made up in a volumetric flask, which has set volumes. I will make up a solution of volume 250cm3, which will allow a maximum of nine titrations plus a small amount of solution to wash out apparatus.

To make up a volume of 250cm3 at concentration 0.1 mol dm-3, I will need to work out the mass of solid sodium carbonate I will need to add to deionised water. First I will work out the number of moles of sodium carbonate in this solution:

I can use this value for the number of moles to work out the mass of sodium carbonate required for this solution:

Indicator Choice

I need to decide which indicator will be most suitable to use in my investigation. Using the information sheet on the use of indicators in acid-alkali titrations, this is a titration using a strong acid and a weak alkali, so methyl orange should be used as an indicator. This is because methyl orange changes colour in the pH range 3.1 – 4.4. The titration curve shown here for a typical strong acid – weak base titration shows that the equivalence point for this titration is below 7 – around the pH range of methyl orange, showing that methyl orange is a good choice for an indicator for my investigation.

Equipment List

This is a list of all the laboratory equipment I will need for my investigation:

• 50cm3 burette
• 25cm3 pipette and filler
• Safety goggles
• Several small beakers
• Digital balance
• Spatula
• Glass rod
• Filter funnel
• Dropping pipette
• Clamp and clamp stand
• White tile

This is a list of all the chemicals I will need for my investigation:

• Sulphuric acid (H2SO4 (aq)), concentration unknown
• Sodium carbonate solid (Na2CO3 (s)), mass 2.65g
• Deionised water
• Methyl orange indicator

Risk Assessment

I need to complete a risk assessment for all of the chemicals I will use in my investigation, as well as all those that are produced. I need to assess what I should do if they are spilt in the laboratory, on the skin, or in the eyes.

• Sodium Carbonate (Na2CO3)
• Can be irritant to ...