## Thursday, 22 January 2015

### 7.4.1 - Laboratory Activity : Acid-base titration

 Laboratory Activity 7.4.1: Acid-base titration
Aim: To determine the concentration of sodium hydroxide solution, NaOH, using the titration method
 Material: » Dilute hydrochloric acid » Sodium hydroxide solution » Phenolphthalein Apparatus: » Burette » Retort stand » Pipette, 25cm3 » Conical flask » Filter funnel » White tile
Procedure:

 1 25cm3 of sodium hydroxide solution is transferred into the conical flask. 2 drops of phenolphthalein are added to it. 2 Hydrochloric acid is poured into the burette using a filter funnel until the 0 mark. 3 The apparatus is arranged as shown in the diagram. The first reading on the burette is recorded. 4 Hydrochloric acid is allowed to flow a little at a time into the alkali. The conical flask is shake constantly and the colour of the solution is observed. The end point is obtained when the colour changes suddenly from pink to colourless. The reading on the burette is recorded as the final reading. 5 Step 3 – 4 are repeated twice more, using a fresh solution of NaOH each time, this time slowing down at the end point by adding hydrochloric acid drop by drop. 6 This video shows the technique for performing a manual titration with an indicator using a burette.
Results:
 I II III Final reading/cm3 24.90 26.10 27.00 First reading/cm3 0.00 1.00 2.00 Volume of acid used/cm3 24.90 25.10 25.00
Calculation:

Average volume of acid that is used = $\frac{24.90+25.10+25.00}{3}\mathrm{=}25.00{\mathit{cm}}^{3}$

The chemical equation for the reaction between hydrochloric acid and sodium hydroxide
 ○ HCl + NaOH → NaCl + H2O

Number of moles of hydrochloric acid = $\frac{1\mathrm{×}25}{1000}\mathrm{=}0.025$

Number of moles of sodium hydroxide = $\frac{{M}_{B}\mathrm{×}25}{1000}$

From the equation, 1 mol of hydrochloric acid neutralize 1 mol of sodium hydroxide.
 ○ Thus 0.025 mol of hydrochloric acid neutralize 0.025 mol sodium hydroxide $\begin{array}{c}\frac{{\text{M}}_{B}\mathrm{×}25}{1000}\mathrm{=}0.025\end{array}$ $\begin{array}{c}{\text{M}}_{B}\mathrm{=}\frac{0.025\mathrm{×}1000}{25}\end{array}$ $\begin{array}{c}{\text{M}}_{B}\mathrm{=}\text{1 mol dm}\mathrm{-}3\end{array}$
Discussion:
 ► The burette and pipette should be washed with distilled water followed by acid to ensure that water does not dilute the acid. ► The conical flask should not be washed with alkali to ensure no alkali remains in the flask before NaOH is added. ► The end point is the point at which a drop of acid added to the mixture in the conical flask will change the phenolphthalein colour from pink to colourless.
Conclusion:
 ► The molarity of hydrochloric acid used to neutralizes 25cm3 of 1 mol dm-3 sodium hydroxide is 1 mol dm-3