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Urgent A/S Chemistry: Titrations

Hi, I'm really stuck on this question. Could somebody please explain the calculation. Thanks in advance :smile:

Ammonium salts are widely used as fertilisers. Once standard method for the analysis of ammonium salts (except the chloride) is to react them in a solution with methanal, HCHO. This forms a neutral organic compound together with an acid which can be titrated together with an acid which can be titrated with standard alkali. For ammonum nitrate the equation for this reaction is:

4NH4NO3 + 6HCHO => (CH2)6N4 + 4HNO3 + 6H20

15.0g of a fertiliser containing ammonium nitrate as the only ammonium salt was dissolved in water and the solution made up to 1.00dm3 with pure water.

25.0cm3 portions of this solution were then treated with saturated aqueous methanal solution and allowed to stand for a few minutes.

The liberated nitric acid was then titrated with 0.100 mol dm-3 NaOH solution. The volume of NaOH solution used was 22.3cm3. What percentage by mass of the fertiliser was ammonium nitrate? [4]