1. Standardization of sodium hydroxide by ethanedioic acid solution

Treatment for result:

• Molarity of the standard ethanedioic acid solution

Mass of ethanedioic acid= 0.8g

Molar mass of ethanedioic acid= 2× (12+16+16+1) +2× (1+1+16)

                                                        = 126

Number of mole of ethanedioic acid= 6.35×

Molarity of the ethanedioic acid=

                          = 0.025M

• Molarity of the sodium hydroxide solution

  

Number of mole of 25.0 ethanedioic acid= 6.35×÷10

                                                                         = 6.35×

Number of mole of sodium hydroxide = 6.35××2

                                                                  = 1.27×

Molarity of sodium hydroxide= 1.27×

                        = 0.045M

Determination of the concentration of commercial colourless vinegar

Objective:

The concentration of ethanoic acid in different brands of vinegar would be determined. As the compositions of vinegar in different brands are different, the highest concentration of ethanoic acid in the vinegar was said to be the best to buy.

Result:

Treatment for result:

• Concentration of ethanoic acid in the commercial vinegar(ShanTou)

Number of mole of sodium hydroxide = 0.045×0.01681

                                                             = 7.56×

∴Number of mole of ethanoic acid = 7.56×

Molarity of ethanoic acid = 7.56×÷0.001

                                         = 0.756M

• Determine which brand is the “best buy”

∴The Lee Kam Kee is the best to buy as the cost of 1mole of ethanoic acid is the cheapest among the three brands.

Discussion questions

What is meant by “standard solution”?

Standard solution is a solution of known molarity.

What is a “primary standard” and “secondary standard”?

Primary standard is a substance which is so stable in the air that it can become a standard in standardizing other substances, for example, ethanedioic acid and anhydrous sodium carbonate.

Secondary standard is a solution that no actual experiment to determine its concentration was taken out. Its molarity is estimated by calculation.

What requirements must be fulfilled for a chemical to be used as a primary standard in a titration experiment? Why is sodium hydroxide not a good primary standard?

A primary standard should be very stable, would not absorb moisture from the air and has a high molar mass.

Sodium hydroxide absorbs carbon dioxide and water in the air, which makes it cannot be weighed accurately.

What apparatus should we use to measure the following volumes of solutions?

1. 250.0  250  volumetric flask
2. 25.00  25  pipette
3. 1.00  1 pipette
4. 20  measuring cylinder