Advanced titration calculations

So now you are confident with run-of-the-mill, text book titration questions it’s time to tackle the types of question you will face in the exam.

The key is not to panic!

Don’t be thrown because the question is a page long, there are loads of reactions and equations you’ve never seen before and you don’t know where to start. Once you have a method for setting out the information you’ve been given, the path forward becomes so much clearer.

In this video I walk you through some of the scarier titration exam questions from recent A level papers, and then there are a couple of practice questions below.

Practice questions

1. Descaling treatments for irons and kettles commonly contain an organic acid as a key ingredient.  One such treatment states that there is 20-40% by mass of 2-hydroxypropanoic acid present in a packet weighing 25g. 

A student sets out to determine the exact percentage of the acid in a packet of descaling agent. He dissolves the contents of one packet in 100.0 cm³ of water.  25.00 cm³ of this solution is neutralised by 31.60 cm³ of 0.80 mol dm^-3 sodium hydroxide.

           CH₃CH(OH)COOH_(aq)  +  NaOH_(aq)   ⇾    CH₃CH(OH)COO^–Na⁺_(aq)  +  H₂O_(l)

Calculate the % by mass of 2-hydroxypropanoic acid in a 25g packet of the descaling agent.

2. Cinnamic acid, C₆H₅(CH)₂COOH, can be extracted from the spice cinnamon. It is a white crystalline solid that is partially soluble in water. 

A student made a saturated solution of cinnamic acid, and then titrated 40.0 cm³ of the saturated solution against a standard solution of sodium carbonate, Na₂CO₃. 

Na₂CO_3(aq)   +   2 C₆H₅(CH)₂COOH_(aq)   ⇾    2 C₆H₅(CH)₂COONa_(aq)    +  CO_2(g)  +  H₂O_(l)

The standard solution of Na₂CO₃ was made by dissolving 0.0665g of the anhydrous solid in 100.0 cm³ of water.  15.75 cm³ of the standard Na₂CO₃ solution neutralised 40.0 cm³ of the saturated cinnamic acid solution. 

Calculate the solubility of cinnamic acid in g dm^-3, giving your answer to 3 significant figures

Answers