Cambridge International AS & A Level Chemistry
Safety considerations • Make sure you have read the advice in the Safety section at the beginning of this book and listen to any advice from your teacher before carrying out this investigation. • Eye protection must be worn at all times in this investigation. • The hydrochloric acid is an irritant. • Methyl orange is poisonous. If you get it on your skin, wash it off immediately.
Method
E
Part 1: Making up the solution of the mixture
1 Weigh out 1.90–2.10 g of the mixture of sodium hydrogen carbonate and sodium chloride.
Weight of mixture …………………….g
M PL
2 Dissolve this solid sample in distilled water and make up to a total volume of 250 cm3 in your volumetric flask as described in the Skills chapter.
Part 2: The titrations
1 Titrate 25 cm3 samples of this solution against the standard 0.100 mol dm–3 hydrochloric acid. Use methyl orange as the indicator.
2 You should look back at the Skills chapter for how to do this.
Results
10
Complete Table 1.2.
Rough titration/cm3
First accurate titration/cm3
Second accurate titration/cm3
Third accurate titration/cm3
Final burette reading/cm3
SA
Starting burette reading/cm3 Titre/cm3
Table 1.2
Data analysis
a Identify the concordant titres and give the average of these values.
Concordant titres = …………………cm3 and …………………cm3
Average of concordant titres = …………………cm3
Using the data you’ve collected, you can calculate the number of moles of the sodium hydrogen carbonate present in your sample. You can then calculate the mass of this compound and from that, the composition of the mixture.
The equation for the reaction between hydrochloric acid and sodium hydrogen carbonate is shown below. NaHCO3(aq) + HCl(aq) ➝ NaCl(aq) + CO2(g) + H2O(l)
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