Dr. Sweeney is research chemistry who recently discovered an old bottle labeled

Question

Dr. Sweeney is research chemistry who recently discovered an old bottle labeled perchloric acid (HClO_4) in the back of his research laboratory. After verifying that the contents of the bottle were indeed HClO_4 Dr. Sweeney was interested in determining the concentration of HClO_4 in the bottle. To determine the concentration of HClO_4 Dr. Sweeney performed a titration using 20 mL of HClO_4 and phenolphthalein. Dr Sweeney determined that it took 24 mL of 0.040 M NaOH to reach the endpoint of the titration. What did Dr. Sweeney observe to indicate the endpoint in his titration? Write the chemical equation that represents the chemical reaction that Dr. Sweeney was studying. What type of chemical reaction is this? What is the concentration of HC10_4 in the bottle that Dr. Sweeney found?

Explanation / Answer

a) Dr. Sweeney observed the end-point of the titration. Both HClO4 and NaOH are colourless solutions. However, when there is excess NaOH, phenolpthalin shows a bright pink colour. The end point of the titration is marked by a sharp pink colour when an extra drop of phenolpthalin is added after the acid-base neutralization has occurred. Dr. Sweeney observed the pink end point.

b) HClO4 (aq) + NaOH (aq) ------> NaClO4 (aq) + H2O (l)

c) The reaction is a neutralization reaction or acid-base titration reaction.

d) As per the balanced equation in part (b) above, the molar ratio of HClO4 and NaOH is 1:1.

Therefore, moles NaOH added = moles HClO4 neutralized.

Moles NaOH added = (volume of NaOH added in L)*(concentration of NaOH in mol/L) = (24 mL)*(1 L/1000 mL)*(0.040 mol/L) = 9.6*10-4.

Moles HClO4 neutralized = moles NaOH added = 9.6*10-4

Concentration of HClO4 in the bottle = moles of HClO4 neutralized/volume of HClO4 added in L = (9.6*10-4 mol)/[(20 mL)*(1 L/1000 mL)] = 0.048 mol/L (ans).

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