How should you break down and approach a tricky question in Chemistry?

How should you break down and approach a tricky question in Chemistry?

 

HSC Chemistry is a difficult subject. There is a lot to learn and remember and the tests don’t make it any easier. Many students struggle with how to answer some of the longer, more complex questions so what we have decided to do here is breakdown an old (but still very relevant) question and show you how we approached it, so you can do the same for your next hard chemistry question.

 

 

2011 HSC Chemistry Paper

Question 26 (4 marks)

A manufacturer makes lemon cordial by mixing flavouring, sugar syrup and citric acid. The concentration of the citric acid is determined by titration with NaOH.

The sodium hydroxide solution is prepared by dissolving 4.000 g of NaOH pellets in water to give 1.000 L of solution. This solution is standardised by titrating 25.00 mL with a 0.1011 mol L-1 standardised solution of HCl. The average titration volume is found to be 24.10 mL.

To analyse the lemon cordial 50.00 mL of the cordial is diluted to 500.0 mL. Then 25.00 mL of the diluted solution is titrated with the NaOH solution to the phenolphthalein endpoint.

The following data were collected during one of the analysis runs of the lemon cordial.

Titration #1 volume 26.55 mL
Titration #2 volume 27.25 mL
Titration #3 volume 27.30 mL
Titration #4 volume 27.20 mL

 

b) Determine the concentration of citric acid in the lemon cordial

 

Our Breakdown!
Step 1

First, read the question properly. This means understanding EXACTLY what the question is asking you for. In this question you should understand what is happening in the question; what is being added to what? Was anything diluted?

 

Here, NaOH has been titrated with standardised HCl solution. This means we can determine the concentration of NaOH.

 

Note (for the purposes of HSC): the point of titration is to determine the unknown concentration of an acid or base by titrating it with a base (for an acid) or an acid (for a base) of known concentration. 

 

The cordial (you can just think of this as a solution containing citric acid) was then diluted and a sample titrated with NaOH. Since we know the concentration of NaOH, we can now work out the concentration of the citric acid in the sample. It is important to note that the cordial was diluted since this would decrease the concentration of citric acid and keep in mind we need to account for this later.

Step 2

After reading the question properly, the next thing we do is calculate the concentration of the NaOH solution. We do this by first writing out the chemical equation for the neutralisation of NaOH with HCl. (This is something you should always do when doing mole calculations.)

 

When writing chemical questions ALWAYS remember to balance and write states. If it is dissolved in water, then it is aqueous. No need to write states for electrons in half equations.

Since n = cV and we know what V (Volume) is, to work out the concentration of NaOH we need to work out the number of moles of NaOH. We note that the mole ratio of NaOH to HCl is 1:1 (By looking at the stochiometric coefficients of the equation) i.e. 1 mole of NaOH reacts with 1 mole of HCl. Therefore,

Be careful with units, volume should be in Litres.

Step 3

Now we can calculate the concentration of the diluted cordial.

To find the number of moles of NaOH used in the titration with diluted cordial we need to know the volume of NaOH used.

To do this, we average the last 3 titrations since the first titration is a rough titration, and so we can ignore it.

The equation between NaOH and citric acid is:

C7O8O6 (aq)+ 3NaOH(aq) -> 3H2O(l) + Na3C6H5O7(aq)

It’s important you know the chemical formula for citric acid and that it is a triprotic weak acid.

So, the concentration of dilute citric acid is,

 

Since the cordial was diluted 10x we need to multiply the concentration by 10 giving us a final concentration of 0.3541M