Core Practical 3: Find the concentration of a solution of hydrochloric acid

Question 1

phenolphthalein solution is

Answer 1

flammable and toxic

Question 2

bench hydrochloric acid is

Answer 2

an irritant

Question 3

the method for the experiment is

Answer 3

• after rinsing a pipette with water and then NaOH, pipette 25cm3 of NaOH into a 250cm3 volumetric flask and fill to the mark with deionised water
• fill the burette with the NaOH solution from the volumetric flask
• pipette a 25cm3 aliquot of HCl into a conical flask and add two drops of phenolphthalein indicator
• titrate the contents of the flask with the NaOH
• end point is reached when pink colour appears and remains for 5 seconds after swirling
• repeat until two concordant results are obtained

Question 4

Write a chemical equation for the reaction of hydrochloric acid with sodium hydroxide

Answer 4

HCl + NaOH → H2O + NaCl

Question 5

The pink colour seen at the end point fades because

Answer 5

the excess sodium hydroxide reacts with CO2 in the air to make sodium carbonate

Question 6

the reaction between the excess NaOH and CO2 in the air is:

Answer 6

2NaOH + CO2 → Na2CO3 + H2

Question 7

How will the fact that NaOH reacts with CO2 in the air when stored affect the volume of sodium hydroxide solution required to reach the end point in the titration? Explain your answer.

Answer 7

The volume of sodium hydroxide solution will not change because the sodium carbonate made will also react with the hydrochloric acid. Two moles of sodium carbonate react to make one mole of sodium hydroxide which will react with two moles of hydrochloric acid, the same number of moles of hydrochloric acid that the sodium hydroxide would have reacted with.
2NaOH + CO2 → Na2CO3 + H2
Na2CO3 + 2HCl → 2NaCl + CO2 + H2O