Acid-Base Titrations with Indicators
- Acid-base titrations are used to find the unknown concentrations of solutions of acids and bases
- Acid-base indicators give information about the change in chemical environment
- They change colour reversibly depending on the concentration of H+ ions in the solution
- Indicators are weak acids and bases where the conjugate bases and acids have a different colour
- Many acid-base indicators are derived from plants, such as litmus
Common Indicators Table
- A good indicator gives a very sharp colour change at the equivalence point
- In titrations is it not always possible to use two colour indicators because of this limitation, so for example litmus cannot be used successfully in a titration
- When phenolphthalein is used, it is usually better to have the base in the burette because it is easier to see the sudden and permanent appearance of a colour (pink in this case) than the change from a coloured solution to a colourless one
Volumes & concentrations of solutions
- The concentration of a solution is the amount of solute dissolved in a solvent to make 1 dm3 of solution
- The solute is the substance that dissolves in a solvent to form a solution
- The solvent is often water
Concentration (mol dm3) =
- A concentrated solution is a solution that has a high concentration of solute
- A dilute solution is a solution with a low concentration of solute
- When carrying out calculations involve concentrations in mol dm-3 the following points need to be considered:
- Change mass in grams to moles
- Change cm3 to dm3
- To calculate the mass of a substance present in solution of known concentration and volume:
- Rearrange the concentration equation
number of moles (mol) = concentration (mol dm-3) x volume (dm3)
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- Multiply the moles of solute by its molar mass
mass of solute (g) = number of moles (mol) x molar mass (g mol-1)
Worked example
Neutralisation calculation
25.0 cm3 of 0.050 dm-3 sodium carbonate was completely neutralised by 20.00 cm3 of dilute hydrochloric acid. Calculate the concentration in mol dm-3 of the hydrochloric acid.
Answer
Step 1: Write the balanced symbol equation
Na2CO3 + 2HCl → 2NaCl + H2O + CO2
Step 2: Calculate the amount, in moles, of sodium carbonate reacted by rearranging the equation for amount of substance (mol) and dividing the volume by 1000 to convert cm3 to dm3
amount (Na2CO3) = 0.025 dm3 x 0.050 mol dm-3 = 0.00125 mol
Step 3: Calculate the moles of hydrochloric acid required using the reaction’s stoichiometry
1 mol of Na2CO3 reacts with 2 mol of HCl, so the molar ratio is 1 : 2
Therefore 0.00125 moles of Na2CO3 react with 0.00250 moles of HCl
Step 4: Calculate the concentration, in mol dm-3, of hydrochloric acid
concentration (HCl) (mol dm-3) = 0.125 mol dm-3
Worked example
Concentration in g dm-3
A student dissolved 10 g of sodium hydroxide, NaOH, in 2 dm3 of distilled water. Calculate the concentration of the solution.
Answer: