# a level titration questions

My weakness is titration questions and i never get them right, and i really want to because i could bag a good 5 marks.
My problem is i never know when we do 25/1000 or the mean titre divided by 1000 and onwards
Original post by ly2021
My weakness is titration questions and i never get them right, and i really want to because i could bag a good 5 marks.
My problem is i never know when we do 25/1000 or the mean titre divided by 1000 and onwards

To calculate the moles of a substance in a solution, you use the formula

Moles = concentration x volume

Most of the time, the concentration will be given in mol dm^-3 and the volume will be given in cm^3. The problem is that the formula doesn’t work if you plug these straight in as the units of volume and concentration don’t match (one is based on cm^3, the other is based on dm^3).

To get around the problem, you need the volume to be in dm^3. If your volume is in cm^3, then you divide it by 1000.
Original post by ly2021
My weakness is titration questions and i never get them right, and i really want to because i could bag a good 5 marks.
My problem is i never know when we do 25/1000 or the mean titre divided by 1000 and onwards

Guess that you refer to units with your question, is that so? depends on the unit itself that is given. For the volumes the calculation from one unit to another is 1000. As in an example explained before, you need to divide to 1000 when cm³ is given, but dm³ required. If it is vice versa, you need to multiply by 1000.

General calculation is: from to dm³ to cm³ to mm³ it is multiplied by 1000. From mm³ to cm³ to dm³ to it is divided by 1000, because:

1 = 10 dm³ * 10 dm³ * 10 dm³ = 1,000 dm³; 1,000 dm³ = 100 cm³ = 100 cm³ * 100 cm³ = 1,000,000 cm³ and so on.
Original post by Kallisto
Guess that you refer to units with your question, is that so? depends on the unit itself that is given. For the volumes the calculation from one unit to another is 1000. As in an example explained before, you need to divide to 1000 when cm³ is given, but dm³ required. If it is vice versa, you need to multiply by 1000.
General calculation is: from to dm³ to cm³ to mm³ it is multiplied by 1000. From mm³ to cm³ to dm³ to it is divided by 1000, because:
1 = 10 dm³ * 10 dm³ * 10 dm³ = 1,000 dm³; 1,000 dm³ = 100 cm³ = 100 cm³ * 100 cm³ = 1,000,000 cm³ and so on.

oh i meant as in, for example a question like
An iron nail was analysed using the following outline procedure. An iron nail was placed in a beaker and excess dilute sulfuric acid was added. After all the iron had reacted to form iron(II) ions, the solution was made up to 1.00dm3 in a volumetric flask. 25.0cm3 portions of the solution were acidified and titrated with potassium dichromate(VI) solution of concentration 0.0167moldm–3. Results mass of iron nail = 3.54g mean titre = 15.50cm3 The table shows the percentage by mass of iron in four different brands of nail. Brand of nail Percentage by mass of iron A 92 B 94 C 96 D 98 Potassium dichromate(VI) in acid solution oxidises iron(II) ions as shown in the equation Cr2O7 2– + 14H+ + 6Fe2+ 2Cr3+ + 6Fe3+ + 7H2O Determine, using the experimental data, the brand of nail that was analysed.

why did they do 15.5/1000*0.0167 instead of 25/1000*0.0167
Original post by ly2021
oh i meant as in, for example a question like
An iron nail was analysed using the following outline procedure. An iron nail was placed in a beaker and excess dilute sulfuric acid was added. After all the iron had reacted to form iron(II) ions, the solution was made up to 1.00dm3 in a volumetric flask. 25.0cm3 portions of the solution were acidified and titrated with potassium dichromate(VI) solution of concentration 0.0167moldm–3. Results mass of iron nail = 3.54g mean titre = 15.50cm3 The table shows the percentage by mass of iron in four different brands of nail. Brand of nail Percentage by mass of iron A 92 B 94 C 96 D 98 Potassium dichromate(VI) in acid solution oxidises iron(II) ions as shown in the equation Cr2O7 2– + 14H+ + 6Fe2+ 2Cr3+ + 6Fe3+ + 7H2O Determine, using the experimental data, the brand of nail that was analysed.
why did they do 15.5/1000*0.0167 instead of 25/1000*0.0167
The 15.50 cm^3 is the titre (i.e the volume of the solution added from the burette).

Since [Cr2O7^2-] is the titrant (i.e the solution added from the burette - this is what is meant when they tell you the portions “were titrated with potassium dichromate(VI) solution”) and we know its concentration (given in the question), this is the best starting point - one can use the equation moles = concentration x volume.

The 25.00 cm^3 refers to the portions of the iron solution that we do not yet know the concentration of - the whole point of the titration is to work out how much iron there is in one of these portions and hence how much iron there is in all of the solution made. It therefore isn’t useful to multiply it by the concentration of the titrant.
(edited 4 weeks ago)