Worksheet of Back Titration

An experiment was carried out to determine the concentration of a nickel (II) sulphate solution. The experiment consisted of the following three stages:

Stage 1： 25.0 cm³  of  0.503M sodium hydroxide solution was added to 25.0 cm³ of the nickel(II) sulphate solution to precipitate out nickel(II) hydroxide.

Stage 2：The mixture obtained in Stage 1 was filtered and the residue was washed thoroughly with distilled water.

Stage 3：The excess alkali in the filtrate was titrated against 0.251M hydrochloric acid with methyl orange as indicator. 18.5 cm³ of the acid was required to reach the end-point.

(a)    Write the ionic equation for the reaction in Stage 1.

(b)    State the colour change at the end-point of the titration in Stage 3.

(c)    (i)   Based on the titration result in Stage 3, calculate the number of moles of hydroxide ions present in the filtrate.

(ii)    Calculate the number of moles of sodium hydroxide that was added in Stage 1.

(iii)   Using your results in (i) and (ii) above, calculate the molarity of the nickel(II) sulphate solution.

(d)       Why was it necessary to wash the residue thoroughly in Stage 2?    
         

<9 marks>